(aka "The Bottom Line")
One of the fundamental problems with compression is a trade-off between time versus space. If you spend more time trying to compress a document, you can usually do a better job. Similarly, if you want something as soon as possible, you're generally not going to be able to do as good a job as possible. (Sound like your 9-5 job?) Seldom do benchmarks show this trade-off as nicely as DiskDoubler and SuperDisk!. In all cases, SuperDisk! breezed past DiskDoubler in speed. Similarly, in all cases, DiskDoubler crushed SuperDisk! in compression. Within each compression package, the results still held: SuperDisk! "tight" mode only took 2/3 the time to compress than its "tighter" mode, but yielded poorer compression. DiskDoubler "A" also took 1/3 - 2/3 the time of method "B" but only yielded slightly poorer compression. Between packages, SuperDisk! "tight" and "tighter" were about 4-5 times faster (or more) than DiskDoubler "A" and "B", respectively.
I performed all my benchmarks on an SE/30 with a Quantum Pro 105 MB drive under System 7.0. I tested a variety of documents: Text, Database, Spreadsheet, and PICT files. Using these documents as a hypothetical contents for my hard disk, I made a crude estimate of the amount of disk space I'd save for each compression level: SuperDisk! "tight", 27%; SuperDisk! "tighter", 44%; DiskDoubler "A", 49%; and DiskDoubler "B", 54%.
For all you number-crunching junkies, the tables are included at the end of the article.
One of the notable tests was the first one involving text documents. I went through my disk, dumping all my word processor/text documents into a folder. Then I took the entire folder and combined them into a single text file. The difference between the folder size and the single document size was about 200K (I probably missed a few, but not many). This large difference demonstrates one important feature of your disk drives, volume block size. The volume block size for any hard disk you have hooked up to your Mac is the minimum size of any file. In my case, 2K (it increases as your drive capacity increases). That means if I have a document with only a single character in it, it'll take up 2K of my drive, regardless of any compression - and if the document makes use of the resource fork as well, that number doubles to 4K. (The DiskDoubler manual has a nice section on this problem in one of the appendices.) Simply by combining the files into one large file, I saved 200K on my drive, not to mention yielding better compression results. Food for thought if you have a lot of small files.
Just as in the previous article, there's no clear winner. If either one has a feature that's absolutely necessary for you, then your choice will be easy. If that fails you, then you'll have to decide based upon your priorities - whether compression level or speed is more important and what degree of transparency you desire.