Hey Jeff, sorry that you are having RSI symptoms ... do you do any forearm stretching and/or wrist mobility drills? If not, let me know and I'll post some for you ...
As for the mice ... you really need to try different ones to determine what is going to be the best fit for your hand ... sometimes it is better to get the more expensive one if that is the one that really fits and helps versus ending up with 3-4 different less expensive ones. My best advice would be to go to a store where they have lots of different mice to try out (they must have those near where you live). What feels good for one hand might not for another even if they are the same size.