I have both; the 120-300 is a heavy beast but captures gorgeous, stunning images. I can't say what is different, but I actually prefer them to those from the legendary 70-200 f/2.8 non-IS Canon. The color and "pop" just seems better. The 120-300 can be handheld if you're relatively strong. Best price I found was at Sigma4Less.
On a side note, I've often heard that the 120-300 is sharper than the Sigma 300 f/2.8 prime. It also takes the Sigma 1.4x TC with very little IQ loss, which is why I sold my Canon 400 f/5.6.
Fred Deutsch wrote:
Can anyone comment about the importance of the IS fuinction on the Canon 300 2.8? Obviously the used non-IS version is less costly. How important do you find the function? Worth the add'l cost?
Thanks for the suggestion to move from my 5D to a 40D but that probably not realisitc for me. I know it's a wonderful camera, but my next camera will either be the next generation of the 5D, or, maybe there won't be a next camera (I'm pretty satisfied with the 5D).
Lenses -- that's another story. I wanna keep experimenting.