Due to circumstances too graphic and gory to explain here, I came into possession of a Canon 24-105mm that I have no real desire to keep. I currently shoot a 40D with a Tamron 17-50 non-VC, and I also have a Sigma 150mm Macro plus a 1.4x converter and some other various lenses (standard primes, fast telezoom, etc.)
I realized that I'll incur a significant monetary loss if I try to sell the 24-105 outright, so I'm thinking of trading it for another lens that will serve me a bit better, but I'm not sure of what I should get.
Here is the breakdown:
- I find the sharpness of my 17-50 to be almost the same as the 24-105 at the same focal lengths, and IS isn't hugely critical to me (but it is a plus nonetheless).
- I am considering trading the 24-105 for a 17-55mm f/2.8 IS, but I'm not sure if it's a worthwhile upgrade from the Tamron 17-50.
- I am also considering trading the 24-105 for a Canon 100mm L IS or an MP-E 65mm; I love macro, but I don't want it to be the only thing I shoot - I would more than likely end up selling my Sigma and 1.4xTC if I went this route.
- I have no immediate plans to upgrade to FF in the foreseeable future, but I would certainly not mind if the cost to me could be under $1,000 and I could get better ISO performance and cropping headroom over the 40D.
Any experiences or recommendations? I'm open to suggestions! 
