I'm basing my opinion purely on my riding impressions. The GT250 uses the frame from the original GT650 that Hyosung was developing. Then Hyosung decided to build a new frame for the GT650. They used the original frame for their GT250. So the frame is not the same for the 250 and 650.
The frame looks very much like the frame on a GS500 Suzuki...a little cheap-looking, but very effective. It's the only part about the bike that looks a little "cobby."
It makes sense that the GT250 weighs quite a bit more than the Ninja 250...it's more of a full-sized bike with a frame originally intended for a 650. It's one of the reasons the GT250 feels so surprisingly "big-bike" for a 250, along with the added torque.
The fellow who is riding the GT250 here in Minnesota is getting 70 miles to the gallon, I believe he said. He's also had it up to 90 miles per hour. Quite a performer.
I rode both bikes more or less back-to-back. The Ninja felt like a cheap toy bike with a bouncy toy suspension (maybe it wasn't adjusted well) and the GT250 felt plush and controlled, inferior suspension technology notwithstanding. The Ninja was a thrill to wind up to 14,000 rpm. But the GT250 has such a smooth and torquey motor...an extremely quiet sewing machine kind of feel...an intangible quality but definitely real and pleasing.
I can see riding the GT250 on an iron butt ride.......but I'm crazy for small bikes and could see using a Ninja 250 or even a smaller displacement scooter on an ironbutt ride, too.
Test ride the bikes back to back and see what they feel like. Hmm, the GT250 felt like a better long-distance highway bike to me. More room, and a more relaxed torquey motor. Also the ergonomics felt more upright than the Ninja.
Maybe that's just me. Anybody else ridden both the GT250 and the Ninja 250 back-to back?
By the way, Hyosung is a South Korean company, not a Chinese company. They actually have built Suzuki's for years, so there's a good chance that if you have a Suzuki, there's Hyosung involved. Maybe that's why the engine seems so refined.