You've had great suggestions for bikes; I don't have anything to add. I will expand on Kestrel's "take it to track" comment. Leave that out of your decision and chose based upon what you're spending most of your time doing. Unless you're buying sporting Ducati or Aprilia, there's very few bikes you can just "take to track" without extensive preparation and modifications. However, that only affects getting ultimate limit out of bike and going for fastest lap-times possible.
Most of enjoyment I've found is just taking
any bike to track figuring out and riding it to its limits. Doesn't matter if it's Virago-250 or an Aprilia RSV4 Factory, they're all fun! Challenge for me is process of wringing its neck and getting maximum performance. Only difference will be lap-times, however, process and enjoyment for me is same.
