What is the subject matter that interests you in a super-tele? In my case, birds. To really be happy I wanted as much focal length as possible with the highest resolving power possible. I wanted to see feather detail. I started with a 70-200 f/2.8 then went to a 300 f/2.8 and ultimately to a 500 f/4. I really love my 500 but I definitely want to upgrade to the Version II 600 or possibly the 800mm f/5.6 .... I can almost always use more focal length in bird photography.

If you are into other wildlife, like large mammals or sports photography you probably will be happy with a high quality 400mm lens. I have shot a lot of baseball, soccer and football games....500mm is way too much.

I would recommend you get the best glass in whatever focal length you thinks suits you even if you have to save a while....you won't be sorry!