If the screen size and resolution is the same, the pixel pitch (centre to centre distance between two adjacent pixels) will also be the same. This will determine the base viewing distance. The smaller the pixel pitch, the closer you can sit. Consequently, a full HD screen will look less "pixelated" at the same distance as an HD ready screen.
The amount of artifacts you'll see depends on the quality of the source and on how well the manufacturer have implemented the scaling and deinterlacing. This is something that you'll need to look at with your own eyes, it's a complex matter indeed.
Mr K.