amfoto, I agree with everything you say about AF speed, but your wrong about "sharpness"
The entire line of Canon Super telephotos is essentially "so sharp you can't see a difference"
We could argue which is sharper of the two for ever, and even the MTF charts bare this out, regardless of our subjective view, as both lenses are better in certain aspects, and are both so good one can't make a ruling on which has a "better measurements" very easily.
Suffice it to say, all of them are sharper than every other lens Canon makes, with the exception of their siblings in the family.
My point, all the rest of what you say is a good argument to chose the 300mm, this one point however absolutely is not.
Again, it looks like the OP has already made up his mind, and agree with amfoto's advice to rent, but it sounds liek he's already shot the 500mm... so rent the 300mm if you haven't shot with it.