I have the 500 f4 mk1 and with the 1.4 tc on the 7D or 1dmkiv its more then enough for me. It depends on everyone's style though. If you prefer the tripod the 600 will be better but if you like hand holding and are good at getting close to wildlife the 500 is the best.
Found this really interesting in saying how APC sensors can never be good. Just goes to show 99% of people on this site don't have a clue what they are talking about. Who knows the 7D mk2 may turn out really good.