In-game, 720p high will "look better" - but depending on your monitor, scaling 720p up to your monitor's native resolution will make it look pretty bad, and in some cases, cause more input lag. This has gotten better in recent years, but most of my 1080p LCDs look awful running 720p. Worth testing with the hardware you have. Should be straightforward to just change the settings and see which you prefer, lol.
If you can find a 720p or 1366x768 monitor to game on, you're good to go.