I would advise not getting a hard-sided piece for plane or train travel. A hard-sided piece would be amazing, but would keep that for car travel. It would be way too hard to lift, carry, and shove into the overhead. Plus it might get scratched.
LVLoveaffair: You can use a Keepall up to a 55 on the plane. Since it's soft-sided it will be fine. My sister has one and has never had a problem with it.