I only have experience with two cases, so take my advice with a grain of salt.
My Canon SD550 had a Canon case. I really loved it. Didn't have the manual controls I wanted but it was small and travelled well. Lots of fun. But literally withing a month fo diving with it, probably 5 or 6 beach dives, I noticed stuff in the hinges I couldn't get out no matter how long I soaked it. Possibly I could have removed it if I unscrewed the hinge and disassembled the entire thing, but otherwise, those grains were INSIDE the hinge, and not going ANYWHERE without a fight. Case still worked fine, but it was more than a little unnerving to hear a crunchy sound everytime you open and close it. Note that the cases are really hard plastic, and if you know anything about screws and hard plastic, you know that frequent disassembly would kill it in short order, as the screw threads will weaken drastically each time you put new strain on them.
That camera was stolen. (See post on SB in about March or April). My new camera and case is the Canon S80 and the Ikelight shell. Camera has all the manual features I've wanted to start playing with, tho it's not as portable. That Ike case, however, is a frigging TANK. The shell is easily 3 times thicker than the Canon was at it's thinnest point. The back plate comes completely off, and is held on by two stainless steel latches that are very easy to clean. No hinges on the plastic bits. It doesn't travel nearly as well, however, and is not much fun to use out of the water, unlike the Canon.
Make your own judgment. I might actually end up picking up a Canon case if I can find a good price at some point, just for those times I might want to use it on land and when travelling (or on the deck of a moving boat, in the rain, hiking along waterfalls, etc.). But the Ike case is a serious piece of work for underwater.
You'll get good shots with either. Just keep the sand warnings in mind, and be careful with it!