The 24-105 is an awesome lens, and hands down the best single lens to own if you're only going to own one with a 6D -- assuming, of course, that you don't need something else for specialized photography.
But if you're always going to have some other lens mounted to the camera, it's obviously a waste.
Since you're dithering between the two...and, since, by "car shoots" I assume either car shows or portraits of cars, I'd very much suggest the 24-105. Just from the size of the subjects, I'm guessing you'd only ever use the 70-200 at the shorter end of its range, and the 24-105 has that covered just fine, plus it's got all the way out to as wide as you ever want in general-purpose photography. (Wider than that can, of course, be amazing, but it's getting into more of a specialized range that requires a bit of care.)
Horses for courses. If you were shooting football, you'd have a 400 f/2.8 glued to one body, a 70-200 f/2.8 glued to another, and that's about it. A landscape photographer could make a good living with just a TS-E 24 and a 1.4x teleconverter. If you were seriously into the macro world, you might not have a lens that could focus past three feet -- if that.
But if you want a jack-of-all-trades lens, the 24-105 is it. It goes wide; it goes long; it's stabilized; it's got great image quality; and it's not terribly slow (as in, it's faster than any other "kit" lens out there). Indeed, you can get a shallower depth of field with less noise at a the same shutter with a 24-105 on the 6D than you can with the 15-55 f/2.8 IS on APS-C -- and at wider and narrower fields of view, too.
Awesome lens.
Cheers,
b&