I own the 100mm macro, and have used the 105 macro, and I'd say the IQ is about the same. One big thing that makes me like the 100mm macro more (and why i purchased it) is the internal focusing. If you're shooting bugs and there's an element extending at them, it's not exactly going to keep them from flying away (if they have wings).
The 100mm is also light, and it focuses pretty darn quick if you want to "eyeball" it and then make minor adjustments.

