I agree with powers_brent. A good tripod will be essential for city-scape shots at night.I would highly suggest the 17-55 as it is incredible and very versatile for a crop sensor camera as you have. You may be better off buying a good tripod and head instead of a lens. If you are considering a 50mm prime, I suggest the "nifty fifty" (50 1.8). It is incredibly cheap and produces great images. That way you don't lose too much money if you find out a 50mm prime doesn't work well for your needs.