Another option to consider: get the 5DII or 5DIII with the 24-105 kit, and add on to it the 50 f/1.8 (it's a no-brainer) plus one or two other primes that can do something the kit lens can't (wider, longer, faster, macro, etc.) depending on your interest.
The non-L primes are still (often) fantastic lenses, and it's not uncommon for them to best their L cousins in certain departments. For example, the 85 f/1.8 still has outstanding image quality (though, granted, not quite as magical as the f/1.2 L), but its autofocus blows the L version out of the park, making it a no-brainer choice if you're shooting indoor sports. And the 50 f/2.5 compact macro is the slowest 50 of the lineup (though still faster than any zoom), but it mops the floor with the others optically, and it can take up to half-life-sized images. (But autofocus performance isn't anything to write home about.)
That's not the case all across the board, though. The 28 f/2.8 can be decidedly underwhelming, and the 16-35 is a better 20 f/2.8 than the prime.