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I have a Canon 50D and am trying to decide between a used 15-85 for $500 or a used 24-105L for $600. I've read all kinds of reviews, but wanna ask it here. Is it worth the extra $100 for the L lens if I'm not planning on upgrading to FF anytime soon?
In general you can't assume that a L lens is always going to be a better choice for you, especially when using a cropped sensor body. This is not to put down L lenses - they are excellent. However, there are some cases where the EFS lenses are better choices for cropped sensor camera use. The specifics vary, but the choice might come down to best functional fit for your use or even to image quality issues.
I have not used the 15-85mm lens so I cannot offer a direct experience-based comparison. I did shoot the old 17-85mm ancestor of this lens on a crop sensor body some years ago. That was a quite different lens in terms of quality, but the functionality is fairly similar. I have used the 24-105 on both cropped sensor and full frame bodies.
Forge the "should I spend more for the L lens" question for a moment and instead think about how you will use the lens and which lens lines up best with your needs. The 15-85mm focal length range covers everything from borderline ultra-wide angle to moderate telephoto on a cropped sensor body. This covers that range that typical one-lens shooters probably are most interested in. On a cropped sensor body the 24-105 covers a range from sort of "wide normal" (equivalent to the angle of view of a nearly 40mm lens on full frame) to a bit longer telephoto.
What this means is that if your expectations are those of a perhaps typical cropped sensor camera shooter, the 15-85mm range is probably more likely to meet your expectations and work for a larger range of photographs. If your expectations are a bit less typical (you don't care much about wide angle, but you want a bit more reach), then the 24-105 could make a bit more sense. For most people the 15-85 is more likely to be a viable one-lens solution and the 24-105 is more likely to leave them wanting a second, wide-angle lens.