Plenty of reasons why I has to be a 250. I'm also a soon-to-be new rider and have been searching my future bike. Mine has to be under 400cc because where I live we have tiered registration fees, above 400cc and they double and quadruple depending on model!
To answer the original post, I can advise you based on my experience being in the same boat as you: RESEARCH! ask yourself first of all why you are getting a motto license, what type of bike answers to that reasons, and what you honestly see yourself on. Research a few models by reading reviews or getting advice on these forums, and get it down to a short list. At that point, assuming your short list takes your budget into consideration, it all comes down to the test ride.
I have had the CBR-250, Ninja-300, GW-250, TU-250, and YZF-R3 on my list, was convinced the R3 was gonna be it, and am now decided on the TU-250. I am now waiting patiently for next spring to take my course and test ride that bike. If for whatever reason money becomes an issue by then, a used CBR-250 is my backup.