In theory, the Powerplate.
A cross trainer is primarily a cardiovascular training machine, which means your heartrate will increase and you will burn more calories. There is a toning aspect to it, forward action will help tone your hamstrings (back of legs) and reverse action will help your quads (front of legs). For maximum effect, you would need to increase the resistance substantially so the muscles are working harder.
A Powerplate, has no cardio benefit whatsoever and is used to tone and strengthen muscles. Genuine Powerplates contract the muscles 30-50 times per second, so the idea is, it's a more intense workout in a shorter space of time and the benefits should be more evident than using regular weights.
As a Fitness Industry worker, I really think Powerplates are pure hype, I believe at best, users who are inflexible, can improve flexibility using the machine for stretching the muscles. If you want to tone les and bum, then squats, leg presses, leg extensions, and hamstring curls are best.
If you don't have access to a gym, then you can do squats, lunges (static and dynamic) and plie squats.