[masraum]

pros and cons of each.

The speed of the L (f 2.8) would be nice and enable you to catch action if you do much action photography. The L lens is one of the Canon Pro lenses, the image that it gives will be top notch, but in many circumstances the difference is not easy to see. It's 24mm wide end which is not very wide on a 1.6x digital sensor. The aperture would enable you to better isolate your subject from the foreground and background with a short DOF.

The IS on the 17-85 will probably end up giving you a comparable exposure to the f2.8, but mostly for still photos. Trying to capture really fast action wouldn't work as well. The 17-85 exhibits (so I've read) some vignetting at 17mm. You won't have as small a DOF to play with as on the L lens.