Telephoto lens question

[Gator]

This Fall I am planning a trip to Katmai National Park, Alaska to photograph Grizzly bears. I am a amateur photographer. This will be my first major wildlife trip to Alaska. My question concerns recommendations for a long telephoto lens. My camera is a Canon EOS-1 Ds Mark III. Since this is a once in a lifetime trip I am looking to purchase a long telephoto lens. My choice is between a Canon EF 800mm F/5.6L IS USM, a Canon EF 600mm F/4.0 IS USM with a 2X teleconverter or Sigma Super Telephoto 800mm F/5.6 EX DG APO HSM Autofocus. The Canon EF 800mm lens puts a major crimp in my budget due to cost $12,349.00. What do you think about using the Canon EF 600mm with a teleconverter? The cost for the 600mm is $8,650.00. Is there any major noticable difference in picture quality with the teleconverter? Any advice the group can give is greatly appreciated.

[bob_r]

Have you considered just renting a lens? It would satisfy your equipment needs for a once in a lifetime event, but still not break the bank. There are a number of companies out there, but the only one I can recommend is:
http://www.lensrentals.com/rent/canon/l ... rtelephoto
They have a fantastic inventory, great prices, great service and wonderful folks to help you. (They're also less than 5 miles from my house!)

One other question, are you sure you need that much length. The only person that I know of that shoots bears in Alaska on a regular basis is one of the members on Pbase. For most of her bear shots in Alaska, she uses a 300mm f/4 IS lens.
Here's a link to her site: http://www.pbase.com/britestar/wild_gri ... s&page=all

Hope this helps.