Both the 250 and 300 are great bikes to begin on.
What's your plan? Do you want to simply learn the ropes, get 10,000 miles or so on the street and upgrade to something bigger? Or do you plan on keeping whatever you buy for the next few years?
If you're planning on upgrading once you get experience, then you might as well get a used 250R. You can go with the newer-looking model (08-12) or the older-looking model (pre-07). Buy it, ride the shit out of it, then sell it for close to what you paid for it (they hold their value really well).
If you're planning on keeping whatever you buy for the next few years, then you could go with the 300. You get a little bit more power out of it, it looks like a supersport, and the insurance is (probably) close to what it is for a new 2012 Ninja 250 (get insurance quotes online; Progressive, Geico, etc). Just keep in mind: You buy a 300, then want something bigger in a few months or a year, you'll take a big depreciation hit (since it's new, you'll be out for the dealer fees, as well).
One thing to note: you're going to drop it; either in the driveway while motionless or out on the road. Dropping a bike sucks. Dropping a new bike sucks even more.
Before you do anything with purchasing a motorcycle, take the MSF Basic Rider Course. You don't know what you don't know. It'll decrease the likelihood of you killing yourself, and it's actually pretty fun.
Around 3/4 of all motorcycle crashes involve untrained riders; take the course.
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