I think either would be fine... but... I would probably go with the 2 butts simply for the reasons that you will get more bark out of the 2 and there are 3 different colors.. textures of meat in the butt. If you pull one you'll see the differences and I think having the second butt gives you more flavors and textures simply because it is another cut of meat.
Id go for the 2 smaller butt like Pig said more bark and you could even maybe try something differant on the second if you wanted to. Good Luck with the decision and with the smoke and don't forget the Q