I had some experience on minibikes and dirt bikes in my past. The beginner course used the night hawk 250, which seemed almost like a dirt bike in look and feel (at least to the early 80’s dirt bike I grew up on). My current motorcycle is a Ninja 250, and although they handle differently, if you can ride one you can ride them both. That being said, the ex250 has an upright seating position and most 600-650 sportbikes are going to be more forward. You could try the Ninja 500 it is a little bigger than the 250 and has a fairly upright position. Megaspaz can tell you about the sv650 (he currently owns one).
“I am the best I am at what I do, and what I do ain’t nice.”-Wolverine