I'll put my 2.5 cents in. i have both the 100-400mm and the 500mm f/4L IS. i love the 500mm and use it almost exclusively with the 1.4TC. It needs a tripod but I can hand hold it for 45secs to 1 minute. Don't really need that much time fr a few bursts. the 100-400mm I use as a walk around lens but, not that often. Now for an in between price range, why not try a 300mm f/2.8L IS and use it with the 1.4TC for 420mm f/4 or the 2xTC for a 600mm f/5.6 and it is quite light enough to hand hold and carry around. At least for e it is after using the 500mm. The canon 300mm f/2.8 sells for $3600 or so but there is the sigma 300mm f/2.3 no image stabilization for about $2700. The canon may be my next lens for walkaround as with my 100-400mm I usually am shooting at 300mm to 400mm anyways. Just another idea!!