The 500r looks like crap. The thing isn't much faster than the 250r yet it weighs quite a lot more and costs a load more.
I have a 250 that I have modified greatly (puts out over 30bhp) and it fits me perfectly. Sure, for freeway riding you do have to rev the crap out of it, but the inline two cylinder powerplant loves to run wide open. The 250 redlines at 14k and hits the limiter at 14.5k; the power curve drops off at about 13k. Considering the fact that a stock 250 weighs just a bit more than 300lbs wet and puts out ~25bhp, it has a darn good power/weight ratio. Although the stock suspension is made for a 90lb rider.
I've made numerous long-haul interstate trips on the 250 where I keep the speed above 75MPH for hours at a time. 75MPH in 6th puts the tach at about 10k. 10k for this engine is its home. The engine will last anywhere from 40-60k miles before it needs to be rebuilt (or buy a new one for $200 on Ebay).
Don't get anything that has more power than a 500r. You won't learn how to handle it properly. Once you have a year or two of experience, take the bike to a track day and take lessons. Once you're a skilled rider, then move up to a 600+cc bike. I see all of these guys on new Gixxer's and R1's and they only stick around for a couple of weeks before they disappear (either from crashing or they wake up). I find it amusing when I see someone riding one of those bikes when yet he can't even make a simple turn.
No matter what size of bike you buy, remember that taking the MSF course is the single best thing that you can do.
BTW- I've recorded actual speeds of 100mph on the 250 on windy days and 100+ on calm days. Remember that all motorcycles have their speedometers set to ready about 10% high. If it shows 70mph, you're actually doing about 63. Kawasaki claims that a stock 250 has a 1/4mi time of 15.5sec. But, I have a good friend that has a stock bike with a 15t countersprocket (which changes the torque output and makes the bike accelerate slower) and he ran 14sec 1/4mi consistantly. For further information, check out the
Ninja250 Rider's Club. There is a FAQ detailing everything from newbie questions to what parts are needed for rebuilding the engine.