I've had two porcelain types. The first had a plastic pedestal with a porcelain bowl which was very shallow. The shallow bowl created some issues but the real problem was the porcelain bowl just broke apart one time when I sat on it.
We had an Aqua Magic V and were happy with it except the hand flush (more on that below). We wanted to add a Fresh Spa Bidet attachment so we had to go with a model that used residential toilet seats.
The one we have now is an Aqua Magic Style II with a deep bowl and residential toilet seat. The only thing I don't like about it is below the ball valve "stuff" gets stuck and looks nasty when you look inside. Some of it can be cleaned with a toilet brush and the spray attachment.
I've had several plastic in older RVs and really don't see the difference.
I prefer the foot flush. No real reason I just didn't care for the hand flush.
By the way- the bidet attachment is one of the best modifications we have done http://brondell.com/spa-suite/freshspa-dual-temperature-bidet.html
It provides that "fresh" squeaky clean feeling.