Hmmm, tough one. For practicality and long(er) distance comfort, the 650 wins hands down. The fairing for distance and the low pipe for room for bags. For a whiz-bang effect from the peers, the 600 has the ultra aggressive street fighter look that will always garner applause. You'll have to rev it to really use it as intended though, from what I've read.
The 650 is basically a faired Bandit with fuel injection, which means super relibilty and low cost maintenance, a friendly mount with decent low and mid range power, excellent for travelling and sport touring.