Although DiskDoubler and SuperDisk! both serve the same function, how they approach the task differs. When you compress a DiskDoubler file, you'll know it's compressed right away - the icon for the document or application that you just compressed turns into a DiskDoubler icon. Just as your documents you create with Nisus are Nisus documents, documents you compress with DiskDoubler are now DiskDoubler documents. DiskDoubler has created some customized icons for common applications that are usually identical to the original application icons except for a "DD" branded into the icon. For those applications that it doesn't recognize, the icons turn into generic DiskDoubler documents. In many ways, this visual cue is desirable - it's very easy to tell what's compressed and what's not, which is important if you're transferring documents around. In others, it's not. One of the nicest features of System 7.0, in my opinion, is the variety of color icons for the Finder. If you use DiskDoubler and compress your files, you'll find all your documents rapidly become identical. Another drawback is if you view files by name in the Finder. Instead of having a "Nisus document" you'll have a "DiskDoubler document." With the ease of cutting and pasting icons in System 7.0, it would be an improvement if DiskDoubler grabbed the icon for whatever it is compressing and simply added a "DD" in the lower-left corner, or simply added a new icon family to the bundle. This would get rid of my half of my gripe (small as it is). Salient has promised that this limitation will disappear in future versions of DiskDoubler.
SuperDisk! takes a different approach. Instead of changing them into "SuperDisk! documents," SuperDisk! compresses files transparently. If you rename "My term paper" to "My term paper.s", the only difference you'll notice is that the file size will shrink. SuperDisk! avoids my one gripe with DiskDoubler, but falls prey to my first warning - it can sometimes be difficult to tell what's been compressed and what hasn't, especially if you've gone down a couple of levels in a folder that has been compressed.
The different levels of transparency between DiskDoubler and SuperDisk! illustrate a good question: how transparent should compression of files be? One of the things I like the most about SuperDisk! is the ability to rename a folder to "Folder.s" and have any document I drop into it be compressed automatically. It's convenient to be able to automatically save the disk space for articles I save off Usenet, or e-mail messages, etc. On the other hand, there are times when I don't like seeing ".s" appended to every filename. After a while, it makes it look as if I'm running on a PC, heaven forbid. DiskDoubler, on the other hand, by its methodology can't automatically compress new documents for me (though it will compress or expand files dropped on the DiskDoubler application icon), but I can always tell at a glance what's compressed and what isn't. I certainly haven't figured out which method I like the best - a combination of the two of them with certain tweaks would be ideal, I guess.
Other than cosmetics, DiskDoubler and SuperDisk! operate similarly. You can open documents and applications that have been compressed as if they weren't. The only difference you'll notice is the cursor changing to a spinning "S" or a "DD" for a few seconds before the documents open. One notable difference is that the only way to decompress SuperDisk! files is via the extension. No freely-distributable decompressor exists (except for demo versions of SuperDisk!). DiskDoubler will still decompress previously-compressed files even if the extension isn't loaded since double-clicking on DiskDoubler icons will launch the DiskDoubler application (and subsequently launch whichever application created the document). SuperDisk! can't do this because it doesn't change the type and creator of the file, although it's not normally a problem. It might cause some worries if you reboot without the extension (reasonable if you're testing for conflicts), since files will seem corrupted if they have been compressed and SuperDisk! isn't running to expand them. If you do that, absolutely do not save the file! That will destroy it. Instead, quit without saving, turn on SuperDisk!, and reboot. Then the file will be fine again. Ideally, Alysis should add this ability to the SuperDisk! Utilities, a program they distribute along with SuperDisk! and via electronic services. More on that in a bit. Another oddity I ran into was SuperDisk! compressing files that get thrown in the trash under System 7. In general, this shouldn't be a problem for most people, but I did encounter some problems shuffling fonts between suitcases and the trash can, not to mention that I tend to throw things away that I actually need and then have to go and recover them (this happens more often than I'd like to admit). The recovered files are actually compressed, even though neither SuperDisk! nor I was aware of that fact. Renaming the file to .s and then removing the .s solved the problem. In addition some programs may experience problems that show up in conjunction with SuperDisk!. For instance, we found that if SuperDisk! compresses VersaTerm 4.0, each time VersaTerm runs, it wants you to enter the personalization information. Not a serious problem and probably not SuperDisk!'s fault, but a pain nonetheless.
In the features arena, each compression package sports some features that the other doesn't. DiskDoubler has safeguards (including working on a copy of the file and verifying the copy before deleting the original) against data-loss due to system crashes while compressing files. SuperDisk! does not have these safeguards for speed reasons, so if you regularly lose power, you should keep that in mind, or, if you're rich, buy an uninterruptable power supply. SuperDisk! supports password-protection, but DiskDoubler has nothing of the sort. DiskDoubler can create self-extracting archives of any number of folders and/or files, but SuperDisk! only allows you to make a self-extracting archive of a single file (no folders, so it is of limited use). In addition, creating a self-extracting archive in SuperDisk! requires that you go to the Control Panel, click a button, and use the standard file dialog to select a file and then select where to save it. There's nothing wrong with this method except the fact that it's much clumsier than SuperDisk!'s normal method of operation. I'd far rather see being able to add ".sea" to the name of a file to create a self-extracting archive. If I wanted to keep the original file I'd merely duplicate it in the Finder first.
Both SuperDisk! and DiskDoubler each support two levels of compression, one faster, one slower - in SuperDisk! terms "Tight" and "Tighter" and in DiskDoubler terms "A (fastest)" and "B (usually smaller)." DiskDoubler also expands these two options into "Smallest Guess" and "Smallest (Try Both)," the first of which guesses based on the type of file and second of which actually tries both methods and uses the smallest result. Try Both is generally not all that useful. As far as supporting other file formats goes, DiskDoubler can expand StuffIt 1.5.1 archives (and is one of the fastest at expanding those archives), but SuperDisk! ships with a utility that can convert DiskDoubler and StuffIt files on your hard disk into SuperDisk! files. That utility can also recompress files already compressed with SuperDisk! to make them open faster or take up less space by changing the algorithm used to compress the file.
Compressing files on your hard disk does affect other file management actions that you regularly perform, such as backing up. For instance, since DiskDoubler updates the modification date (it is creating a new file, after all), files compressed by DiskDoubler will appear to be different to a program like Retrospect in an incremental backup. This can be a pain if you regularly compress and expand the same files without changing them in real life. SuperDisk! added specific code to avoid expanding files for Retrospect, since right after SuperDisk! expanded a file, Retrospect compressed it again, wasting time. SuperDisk! doesn't change the modification dates on files, so if the only change is that SuperDisk! has compressed the file, it won't appear different to backup programs that check the dates. I don't know how other backup programs will react to SuperDisk!. You also don't really want to compress files in your System Folder most of the time, and both programs have added safeguards to prevent people from trying to compress vital files like the System and Finder.