While some models fold to be quite compact, others aren't as convenient. Since metal is much stronger than plastic, a scooter with fewer plastic parts is more durable than one with many. Also, joints should be welded, not bolted. A weld is much stronger. And wheels should be replaceable.
Most models seem to have large rollerblading wheels, which are available at sports equipment stores.
Hey, this is Street Cents. 8.5 Street Cents testers said the Razor was easy to fold, had a smooth ride and good brakes. The only downside was the high price. Brakes weren't bad. The Phoenix had too many knobs and switches for folding.