Is there a noticeable difference in PQ between the two?

I don't believe so. I think the main differences are small things like number if HDMI inputs and such. There is also a difference in the contrast ratio. I dont' remember off hand what that is. Go to one of the retailer sites that allows you to compare models. That's the easiest way.

The P42X1 appears to have more features. It on sale at BB for less than their price on the P42C1.