If you have plans to upgrade to a full-frame camera in the near future, then the 24-105mm f/4 L IS is the way to go. If not, I'd suggest keeping the 17-55mm f2.8 IS. I find the 17-55mm focal range to be the best general purpose focal range for a crop (APS-C) sensor. Right now, I have 3 zooms and 3 primes--and if I could only keep one lens, the 17-55mm f/2.8 IS would be my personal choice.