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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.
 
The smaller the screen, the better the picture will look. But the smaller the screen, the closer you have to sit to see it. For a computer, sitting a normal distance, the 21.5" is going to look butter, but there isn't going to be a huge difference unless the screen sizes were significantly different. Which they are not.
 
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