You wouldn't buy a black-and-white TV or one with a giant CRT tube in 2014, so why would you consider getting a set with a 720p resolution? These days, the difference in price between a 720p TV and one with a "full HD," 1080p resolution is small, but the upgrade in quality is significant. For example, a 32-inch, 720p Samsung UN32EH4003 goes for $247, while its 1080p brother, the Samsung UN32EH5000, costs $277.
Having more pixels means a sharper picture, no matter the screen size. A 720p screen shows only 1280 x 720 pixels, while full HD provides 1920 x 1080. And today, more and more content is optimized for the higher resolution, with Netflix streaming at 1080p and a number of leading console games, including Destiny, outputting at full HD. Do you really want to be staring at the low-res screen you buy today in three years, when a lot of content is in Ultra HD (3840 x 2160) and everything is at least 1920 x 1080?
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