Before getting my 5DII my 17-40 is glued to the crop bodied 450D. It has provided me with many great pictures. The 24-70mm 2.8 is a great lens but I think would suit a full frame user more. It is sharper with larger apertures than the 17-40. Ah if only the 17-40 is a 2.8 but we are digressing.
I love my 17-40 on the crop. It is even better performer on a full frame for the "ultra wide angle" perspective should you migrate later on. For night photography a fast lens is a must. I would use a prime here. (insert prime choices... dependent on budget) Oh one more thing, the 17-40 is one of the cheapest most affordable L lens out there. Fast focusing and built like a tank. All in all a good solid investment.