I used to be a butterfingers guy, but I think I'm undergoing a paradigm shift...right now it's about equal for me, leaning toward Reeses. But they both have their strong points (and weak points).
Butterfingers are more filling, have better texture, plus they take longer to eat if you're a candy pig like me. On the downside, they are really messy and stick in your teeth.
Reeses are more flavorful and delicate. They are much richer than Butterfingers. Unfortunately, you only get two per pack (four if king size) and they get downed in two bites apiece.
...I can't decide! >_<