« on: December 30, 2012, 03:39:24 PM »
I'm a hobbiest as well and am the owner of a 500mm version I. I initially purchased it in a year I wouldn't be able to travel much and escape the city. Birds were going to be my weekend nature escape. While I enjoy photographing birds since owning the 500 I've found that I really enjoy wildlife travel to photograph all kinds of wildlife. So, you may want to consider if you will only use it for birds or if you would use it for larger wildlife as well. If you want to stick with birds the extra reach of the 600 may be better. However if you want to use it for larger wildlife the 500 may be more versatile.
The 500 is a little smaller and lighter which makes it easier and quicker in situations where you want to handhold it. On a recent trip to Churchill to photograph polar bears, there just wasn't room for a tripod so I would handhold and brace it against the rail.
Since I've owned the lens I've used it on several cameras with different crop factors 1.6, 1.3 and full frame. Lately I've been using on a 5DIII with a 1.4 extender. That size works well for the different wildlife sizes and if I were to upgrade I would stick with the 500. However, if I were stick with birds only, I would opt for the 600.