I use both 500/4 IS and 300/2.8 IS on both crop (7D) and full frame (5DII) cameras.
I would recommend you consider the EF 300/4L IS. I use it too, especially for handheld shooting and whenever I have to trek any significant distance with my gear in a backpack. It gives virtually the same angle of view on a crop camera as the 500mm does on full frame. It's also quite nice on full frame... and it works well with a Canon 1.4X teleconverter, if needed.
EF 300/4 IS on 7D (f4, i.e. wide open)...
EF 300/4 IS with 1.4X II on 7D (f5.6, i.e. wide open)...
EF 300/4 IS with 1.4X II on 5D Mark II (f8)...
The 300/4 with lens hood retracted isn't much bigger or heavier than one of the Canon 70-200/2.8 or the 100-400mm (when that lens isn't extended).
IQ of the 300/4 is surprisingly close to that of a 300/2.8. It doesn't render as nice background blur.
It has a peculiarity where specular highlights sometimes show a magenta tint (see the hawk's eye below), but that's pretty easily fixable in post-processing, if it's a problem at all.
Above was shot with 300/4 (f5.6) on 5DII.
The 300/4L IS is an older design... I think it was one of the first EF lenses to get IS back in the last few years of the 20th Century, and that probably was the last time it was updated (unless Canon did a "silent upgrade", which I've no reason to think they did). But, what the heck, it's still in production and still works quite well! It has typical L-series build quality and sealing, has a built-in lens hood and uses 77mm filters. USM focus is fast and accurate (and IF), it has a focus limiter, and its IS has standard and panning modes. Its IS is not self-cancelling on a tripod, the way the 300/2.8 or 500/4's IS is... so if used locked down on a tripod you need to turn the 300/4's IS off manually... not a big deal. Besides, I bought it primarily for handheld shooting and rarely put it on a tripod (though the lens comes with a tripod mounting ring), can't recall the last time I turned off IS on it.
300/4 IS isn't cheap, but isn't pricey either.... I picked up my copy used but looking like new, for well under $1000.
Oh, and it's also the closest focusing of all the Canon super teles over 200mm focal length. It's popular with Canon shooters who photograph butterflies a lot, for that reason.
EF 300/4 IS on 7D, near minimum focus distance (f4)...
We're all waiting to see how 7D Mark II performs. There are some initial 7DII image tests available for comparison over at the Imaging Resource (select cameras to compare from the drop-down menus at
http://www.imaging-resource.com/IMCOMP/COMPS01.HTM
). To my eye, it looks to improve on 7D high ISO performance by a stop or maybe a little more. The AF system sounds exciting, too... Otherwise the description makes the Mark II sound like a nice, solid upgrade to my two 7Ds, now 5 years old with over 100,000 clicks apiece.
