Personally I prefer the bass. It's more consistent musically than the guitar, common in more genres, and it's great to learn the mechanics of intervals and the role the bass has. With the guitar you have that stupid smaller interval between G and B...which does make doing chords easier, but it screws with your intuition. Basically, if you want to make music more easily and go to open mic night and screw around, the guitar is great. If you think you're ready to really dig into the meaty theory, you want to play with other musicians, and you don't mind not being able to just strum your troubles away, go for the bass.

If you can swing it, I also recommend getting one of those RadioShack keyboards that has the automatic chord thing on it and it displays what chord is being played (and more than just the basic M, m, 7, m7 chords, see if you can get one that shows augmented, sus4, and inverted chords). Mess around with it as much as you can. Listen to what happens when you use different chords. The hands-and-ears-on approach will teach you way more effectively than any book can.