Been riding mtb for 25+ years so have used thumb shifters, Shimano RapidFire, SRAM trigger and SRAM gripshift. My favorites are still the gripshift. They make a version to mate with Shimano 2:1 ders and there's the SRAM 1:1 pull version (rear der). I've got issues with my left thumb/wrist and I still stick with 'twisties' over triggers. The lower level SRAM triggers are a bit 'heavy' in their action. I don't have overly large hands and am happy with gripshift.

I've taken the grip portion off some SRAM Attacks in the past to use on the SRAM XO shifters. They have a little less 'knob' to them. SRAM may sell those separate and then you could combine that smaller shifter grip on the XOs. Think those smaller grips are called Amy from the Shorty shifters. See if your dealer can order up those replacement shifter grips. I know you can buy the XOs that way.

The other sweet combination with the gripshifts is to use the ESI Chunky grips (rather than the stock SRAM stuff)
http://www.esigrips.com/Silicone_Gri..._Absorbing.htm

If you're up around mid-Michigan before you are locked in you're most welcome to try mine. Likely be on the pavement as the trails are all under snow ; )