Not all 600''''s are equal
If you dig a bit through your back issues of Motorcyclist, you'll see they did a semi-scientific study of how the whole equation of rider skill + bike prowess.
Here's the nickle summary: increasing rider skill makes the bike go way faster around the track than changes to horse power, torque, or anthing else. However, the difference between lap times from bike to bike varied more for unskilled riders than skilled. Got that? So if your goal is to go fast, it is critical that you choose the right bike if you're unskilled. If you've got the skills, you can ride around the bikes faults.
That said, unless you're at the track, speed isn't that important so look for something that is comfy, affordable (insurance too), and can do all the jobs you want to do (backroads, commuting, sport touring, etc).