The standard answer is that you want to feed your display with it's native resolution and avoid all processing at the display level (i.e. you get the 1:1 pixel map and maximum sharpness). So for you and your 720p display 720x1280 is perfect and for someone with a 1080p display 1080x1920 would be perfect to achieve this. Obviously lower resolutions require less graphics power and less video card all else held equal.
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