Actually the difference is really noticeable when working with big-sized files or hundreds of files. I have just tried ZIPping and RARing the USA-Europe part of the GoodSMS set (v2.01), which is 92,326K in size unpacked. Here are the results (making one big zip and one big rar archive):
ZIP -> 46,244K
RAR -> 43,894K
About 2.5MB spared, which is nice, but when packing smaller things, the results are not that surprising. For instance a genesis game: Alladin (E) [!] - 2048K
ZIP -> 1159K
RAR -> 1085K
The opportunity to use rar archives is pleasing, though greatest differences occur when packing data that wouldnt fit on a whole smc. Anyway, this was just to prevent questions like "Whats the difference between the compression ratio of ZIP and RAR?"