I've played in a drummerless band, it's a great outlet. My guitarist was an acoustic rhythm player so I had a good progression and rhythm to move through. With a choice, I'd choose drummer over a guitarist though.
Drummer. It just seems like it is easier to communicate grooves with a drummer. But hey, I'm a music whore, I'll jam with anybody. Well, at least once anyway. Joe.
I play in duos with both formats fairly frequently. The one with the guitarist gets a lot more gigs, but the one with the drummer is a lot more fun. My main gig is a jazz trio featuring neither one, and I really enjoy that.
It's a lot easier to jam with a drummer, and in many ways more fun, you don't need to worry about harmony and melody and you can just rock out however you feel. In that regard I think it's a lot more braindead and immediately satisfying. But, there is something to be said about sitting down with a good guitarist and burning through standards or just playing around with chord progressions and hearing. It takes a lot more skill I think, to jam with a guitarist and keep it interesting.
Probably...drummer. That is, unless its my friend Rach...cuz for soem reason, I always find that interesting.