I use Canon lenses on Canon bodies, so YMMV, but I shoot long lenses, out to 1000mm focal length. I'd suggest going with the 300/f4. It's a lot less load and will AF very well on the Canon bodies, even with the 1.4x teleconverter attached. Weight is an issue when hiking after shy birds and animals.
The only real benefit of the 300/f2.8 II over the 300/f4 is that the IS is good for 2-more stops. 4 vs. 2, which is very real if you, like me, hand hold almost everything. The comparison of the two lenses shows no difference in IQ:
http://www.the-digital-picture.com/R...mp=0&APIComp=3
Consider buying used. The Canon super-telephoto lenses hold their value, after the first small burst of depreciation.
Dave