How does the EX500 compare to the EX250?
The Kawasaki Ninja 500 (EX500) was discontinued after the 2009 model year, but there should be used ones for sale for many years.
The 250 is a screamer that performs well and can easily get you into trouble. Common complaints about the 250 are excessive nose dive when getting on the front brake hard and skittishness of the rear during high speed cornering. Both are easily corrected with suspension adjustments. A bit more wind protection would be great, too. This can also be changed.
The 500 addresses the diving of the front end (to a degree), the rear's skittishness, and the wind protection. A plus for the 500 is the greater torque and power off the line. It pulls stronger and will get you into illegal speed territory quicker than the 250. With greater weight, it has slightly more stability in high speed and windy situations. Insurance is just about the same for both, with the 250 getting the nod for gas mileage.
Service requirements are almost identical for both as well. Being that they're both parallel twins, the technology is the virtually the same. The 250's brakes are better tuned than the 500's, and the shifter is smoother.
Downfalls of the 500:
From a comfort standpoint, the 500 is probably better suited for a larger rider. If this is an issue, go spend some time on both and see which one is more comfortable for you.
Bottom line, both bikes are great. You'll just have to figure out which one is for you.
While we wouldn't consider these as proper bikes for beginners, here are some other bikes that you may wish to look at in the bigger-but-not-sportbikes category, if you're looking to move to something with a little more displacement.