I have the RK Imperial (OK, an early version) Decapod that I bought for a yet-to-be-done bashing project, and I have seen the Premier version.
No comparison, really. The RK is a presentable little loco, and better than no PRR Decapod if you want one and need to make it fit.
However, if your budget and track curves allow it, go for the Premier. It's more money, but it's also no contest, at least in appearance.
I believe that the modern RK locos are electronically nearly identical to the Premier, so the performance should be very close.