There is a huge difference between a 250r and a 650r. The 650 weighs (wet) about 80 lbs more than the 250. It has more than double the HP and torque. It will go 125-130 honest mph (not speedometer reading). It will do about a 12.6 quarter mile, whereas the 250 will do a 15.1 or 15.2 quarter mile. Mid-range torque will feel about triple the amount on the 650. The 650 sits higher, has a smaller gas tank, gets worse gas mileage, and thus has a shorter range than a 250r. Few bikes will match a 250r's range (old Kawasaki Conquors with 7.5 gallon tanks will and some old BMW's). If you can ride a 250r well, you CAN ride a 650, you should just take it slower and easier till you get used to the increased power. There is also a huge difference between a ZX-6r (600) than a 650r. The 600 will do high 10 second quarter miles and will approach 160 mph in top speed. It's lighter than the 650, sits lower, gets worse gas mileage will cost more to insure (with most companies), handles better, has a better suspension system, tires cost more, don't last as long, and the seating position is better for racing on a track, and worse for driving long distances on the street (lower bars, further back footpegs, higher footpegs, etc. You could ride either one, just respect them both. The 600 will cost about 33% more too.