For my 2 cents.. I've used multiple copies of the 28-135 whether it was me owning it way back when to using it one clients cameras, I've never had a crisp sharp photo with this lens... Dont get me wrong, it's good for a general purpose lens and for most applications, it's acceptable, but It has always kept me wanting more... Quite frankly I never have been impressed with the quality of the lens... The 24-105, while "only" an F4, with the improved qualities of the newer cameras ISO's, F4 is becoming quite usable. If i were in your shoes and had to ditch one lens, I'd sell the 28-135 for whatever I can get for it. Comparing the two lenses, there is no comparison... Some of my clients have 50d kit cameras they insist that I use (their liability if anything happens, not mine) and I can show them apples for apples all day long how much crisper and sharper, at 100% views, it becomes. Ditch the kit lens, keep the L...