I never owned a 250 Ninja. I have owned Rebels both 250 and 450, a 71 CB 350, an 87 Yamaha SRX 250, and the Suzuki TU250 X Vulcan 500, CBR 250 R (2011). I am 6 feet, weigh between 200-210 and 62 years old. A major consideration for me is riding position with the "standard" being my favorite, because it does not make my back hurt until I have covered probably over 100 miles, then it's time for a break.
Never rode a 250 Ninja but my understanding is it needs to be revved high to run decently.
The Ninja 300 pulls fine from 3k RPM in every gear. I have never rode a bike with a 13k redline but this Ninja 300, in my opninion is a quantum leap above any of the previous 250 CC bikes including the CBR250R, one of which I owned a rode several thousand miles last year (retired in my garage).
With a gross weight of about 580 for me and you being about 90 pounds heavier, I would think you would find the Ninja 300 to be profoundly better than the 250. The low speed torque is the secret to the improvement.
I can sit up straight on the Ninja, the postion is almost identical to the 71 CB350 and the Suzuki, in other words more of a standard seating position in a bike that looks like a sport bike.
Just be patient, I had been watching for a used Ninja 300 for months on craigslist, knowing that bike riders think they need more bike when they get some confidence.
I saved about a grand compared to buying new from a dealer. The taxes, fees, tags and a years insurance (no collision), cost me $253 over the $4k I paid for the bike.
After riding it for one day, I think I will sell all the other bikes except for the 71 CB 350 which has nostalgic value. Its funny how close the old CB and the Ninja are in performance, until you try to stop with those 42 year old cable operated drum brakes!
First bike I ever owned, 40 years ago was a Kawasaki 900 Z!. I got over the hyperbike phase on that one. Being where people did not expect you to be is the road to an early grave.
regards
Mech