You should get a 250. They make great first bikes and are still crazy fast. The Ninja can top 100 pretty easy, though you'll find you have more fun going 30-40 on windy roads blasting through corners than you'd have going over 100 getting buffeted by wind and being blown around. They're also much less expensive and resell much better. If you buy a bigger bike for 8 grand, you'll be lucky to resell it for 5, whereas the 250's can be found under 5 and resell for almost what you bought them at. If you do upgrade that makes the 250's a good idea. If you've got friends with bigger bikes, you'll still be able to keep up most of the time.
That said, the ninja 5's are gorgeous and killer fast and fun.
That said, the ninja 5's are gorgeous and killer fast and fun.