Basses are a lot harder to play than guitars! Like Bryan said, try playing a guitar part on your bass (use a six string tuned an octave lower than the guitar if you like). It's a little harder, isn't it? Basses are pretty obviously harder to play. "But guitars have so many difficult techniques," people always say. Try them on a bass then. Both are constructed in the same way, so you can perform the same techniques on them. In most cases, the bass will be harder to play. So there you go. Who's got the more difficult instrument now?
Which instrument generally has the most difficult part to play is a better question to ask, but it's still just a dumb contest. "Who's the most talented instrumentalist, me, or my guitarist buddy," is the question you're really driving at. The "difficulty" of the instrument always gets caught up in it. To a sane person, being easy to play is a good thing. Forget about proving how great you are by convincing everyone that you're doing something hard. Masochism != greatness. You play the bass because you like it, right? What else matters?