I've had both cars and they are both good. It depneds what kind of surfaces you are racing on. For high grip I would definitely say the B6 is a better car.
I had the XB2 carpet and Dirt edition but for me the dirt car was far too lazy and slow (seems to be the general thing with modern dirt cars). I found the carpet car worked very well in low to medium grip conditions but got slower on higher grip surfaces.
I wouldn't say there's much in the build quality. The B6 is a very tough car but maybe the XB2 is a bit over engineered in comparison. Thinking about it I actually broke more on my XB2 than my B6. One thing I don't miss about the XB2 is the terrible gear diff that constantly leaked. You really do need to check that every single meeting....even mid meeting if you've done a bit of practice. It's well worth replacing the diff cap and gasket frequently as it seems to warp.
Both are good cars and I'm sure you'll be happy with either
