Which 600cc Sports bike do I buy?

Go to some bike meets, talk to the guy and girls, if possiblle (ask them first) sit on the machines and make sure that they fit you.

Great question, but I need to ask you another: What type of riding will you be doing the majority of the time? If it's recreational sport or racing, then i recommend the honda CBR and the Suzuki GSX-R. These bikes are the easiest to repeat hard laps on and are great all-around speedsters. Yamaha R-6's in the 2003 year were late to the party on some design tweaks the Honda and Suzuki were on top of. For instance, inverted front forks and radially-mounted full-floating front brakes. Yamaha's also had some suspension issues when ran extremely hard. They had a tendency to soften up when heated up.
If casual riding for longer time periods sounds more your bag, do not forget to look at the Triumph TT600 and the Daytona 600, it's predecessor. These bikes are down on power (barely), but the softer standard suspension settings allow for easier handling on longer durations while still enjoying the performance that the 600cc supersports class has to offer. Ultimately the choice is yours, so make sure you're happy with it! Best of luck.

I would pick the GSX-R on looks alone: Motorcycle USA website has a yearly "shootout" between the 600's: check it out.