Definitely the 24-70 f/2.8 if you're looking for a super versatile lens (a lot of people recommend this as your first lens). The only reason I'd get the 24-105 f/4 would be because of the range it offers. If you know you'll be shooting in low light situations then the 24-105 f/4 can't help you very much, where as the 24-70 f/2.8 let's in a lot more light and can really help capture images when there isn't that much light present. Overall the 24-70 f/2.8 is the way to go but if you want to save some cash the 24-105 f/4 won't disappoint.
Note: If you know you won't be shooting in low light that much, I think you'd be better off purchasing the 24-105 f/4 and saving some cash.