Christian...
Now that you seem to have decided on a 300mm....You mention that a friend sold his 300 2.8 to get the 200 1.8, but have you considered buying a used 300 2.8 IS? Just to be clear, the 300 2.8(which is what you said your friend had)was made from 1988-99, and the 300 2.8
IS was made from 1999-2010 AND not only has IS, but also better IQ compared to the original.(and lighter)
I mention this because you can easily find the 300 2.8 IS version used in very good to excellent condition for about $4000...which is a lot less than $6800..and, if you look at the date code on the lens, you might be able to find one that is only a few years old since they were still making it in 2010.
I like Bryan's review over at the digital picture...
http://www.the-digital-picture.com/Reviews/Canon-EF-300mm-f-2.8-L-IS-USM-Lens-Review.aspxGood luck!
