This is my first-ever post, so thanks in advance to anyone who wants to respond.
After much deliberation, I'm pretty much sold on the Canon 100-400 for birding/wildlife/long distance shots. I've tried the 300 and 400 primes, and I simply prefer to have zooming capabilities. I also prefer the weight and performance of the 100-400 to any of the long-range Sigma zooms. So no advice needed there.
However, I am looking for some advice on what to do in my own particular situation. I've heard about soft vs sharp copies of the 100-400, so I bought a brand new copy from a local store on the basis that I could take it back within a week for a full refund if it turned out to be soft. Luckily, I was then able to find a used copy (built way back in 1999) at another store that I was able to buy on the same "return within a week" basis. I've now tried them both for several hours, focusing on objects in my back yard at 400mm. And without a doubt, the 1999 version is sharper at 400mm, most notably in the center of the frame, than the brand new copy. I have a dozen side-by-side examples to prove it, and it's the same every time. (I use a T2i, BTW, so I don't have any micro-adjustment capability.)
So...seeing that I don't want to compare lenses forever and seeing that not every store will allow full refunds (and seeing that there isnt an endless suply of 100-400s in my area), I'd like to make a decision. My question then: Would you buy a 1999 100-400 for $1200 that seems to work like a charm in all aspects and looks, as best as I can determine, clean, or would you pay nearly $1700 for a brand new version that isn't as clean at 400mm but could be returned to Canon during warranty (I'd do this immediately) for calibration/repair?
I know this is my own decision, but I thought I'd try to solicit a few thoughts first. I'm still fairly new to high-end camera stuff. Thanks again to anyone who wants to repsond.