@jimp:
The x86 version sees more testing, due to its wider user base, but these days there isn't much difference between them except the architecture. You probably can't access all of your RAM on x86 though, so x64 may be the way to go.
I confirm… I was also setting-up CARP between a x86 and x64 version of pfSense 2.0-RC1 (both latest build)... no problems at all... wow...