Skip the Ninja 250 - too small, physically. At least for me - 5-10 and 200 lbs. And I just don't really get off on revving to 14,000 rpm. Although, I really DID like the 75 mpg!
Skip the GB500 - too pricey.
Go for the Ascot 500 single! I've owned two of 'em - electric start, disks at both ends, TONS of aftermarket support and a great forum over on Yahoo!Groups. All the little maintenance tips to correct the minor flaws they DO have. And 60 ~ 65 mpg, to boot.
TONS of fun, and nothing bland about 'em (like the GS500s)
Good luck with, however you go.