Pot A is a better match from a aesthetic point of view. I really like the way the surface matches the bark of the tree. However, Roger is right - it is just a smidgeon on the small side, so Pot B is a better fit.
I like both pots, especially the surface treatment of pot A. However, pot A looks too small for the tree and the whole looks very unstable. Pot B is just as good looking, and has the advantage of looking stable.