Seeing that they both use the same panel tech, and their backlights are the same, the only difference is going to come from pixel peeping. The 21.5" has a 1920 x 1080 display (same as the movie you're playing), while the 27" is 2560 x 1440, which means the movie will have to stretch to fit the screen.
If you sit further back, the 27 is going to look better purely because it's bigger.