No reason why you can't use either of those lenses on your 450D. The only thing with the 24-105mm is it isn't very wide on an APS-C camera. Really depends on what you shoot.
If you're replacing an 18-55mm kit lens you'll be impressed with the quality but not with the wideangle coverage. On a 450D, the you get 63.3 degress (horizontally) at 18mm and this drops to 49.6 degrees at 24mm. Thats a loss of 14 degrees angle of view on just 6mm of focal length.
It depends on how much of an issue that is. If you don't use the wide end, or you've got a separate wide angle lens, then don't worry about it.