Richard Hatch , May 29, 2012; 08:18 p.m.
I have a 300 2.8 and use it alone, with a 1.4X and a 2X. I have not used a 600 f4. Having looked at many images with a 600 f4 I would say there is a very noticeable difference. Hands down the 600 f4 wins except in the cost department (and it won't shoot at 300mm :) ) In the "Good Better Best" the 600 is the best while the 300 + 2X is Good.