"I had a look at some screen-shots from Prepar3d and I have to say I wasn't impressed. The ones that stick out in my mind are of a 747-400 and it looked like a plastic toy  it may run smooth but if the models look like that I'm out."

Prepar3d is just updated FSX and graphically its not any different from FSX. Maybe that picture was taken with Prepar3d ran on low graphics settings or just some bad freeware addon model, but sim itself is just more stable FSX at the moment.

So you can basically add all your addon aircraft and sceneries to it from FSX and they should look and work just the same way I believe, though some complicated addon airplanes could have problems, not sure.

Personally I am not going to switch to Prepar3d until it offers enough advancements from FSX and still stays compatible with most addons made for it.