The differences depend:
If you compare a bolt-on guitar to a bolt-on bass, I'd say the neck maybe takes longer, but is not nessesarlly(sp?) more difficult.
If you compare set-necks, they are fairly the same, the main difference, and difficulty will be with thru-neck basses.
I've built both, I can't really say which is harder, though I can say the basses I built were funner, with bassists being more open the the use of exotic woods, exotic body shapes, plus I have found laminated necks to be cool looking, funner to make, and stronger.