Hello there cbhiii and welcome to the Cameralabs forums!
It sounds to me that you're after a good quality general purpose lens, so I'd think carefully about the range you're after - remember your 400D / XTi will effectively multiply all focal lengths by 1.6x, so the 24-105mm will act like a 38-168mm and the 24-70mm will act like a 45-112mm.
Neither will offer true wide angle on your body, so that's what you'd be sacrificing by going for them. They are both great quality lenses though!
If you do want a general purpose lens with true wide angle for your body, you'd be best off going for something which starts at the 17 or 18mm focal length. The EF-S 17-85mm is an automatic choice, but if you're after a step-up and like the sound of f2.8 aperture, then the EF-S 17-55mm f2.8 may be a good bet.
Check out these articles if you've not read them already:
http://www.cameralabs.com/features/Cano ... s_upgrade/
In the meantime, Cameralabs forum member Bob Andersson went for a 24-105mm to go with his 400D and seems delighted with it!
Bob, are you still convinced it was the right choice, or are you missing wide angle coverage?