You really need to search this forum
There have been dozens of similar threads on plasma vs LCD/LED.
That said, I will repeat what I have said previously. Plasma is capable of providing a superior picture, provided you have adequate light control.
I would go up one level though from the U50 since that's where the Panny's start to shine, PQ-wise. You were probably referring to the UT50, which includes the fast switching phosphors/infinite black panel technologies. I'd steer you up one level to the ST50 if your budget permits though. A much better panel, but in a very controlled light environment, the UT50 could suffice.
Tell you daughter that 3D still is really very toy-like and not worth worrying about on a home flat panel. Future version of 3D will be much, much better.