720p is technically HD, but that's the lowest HD standard. 1080p (or 1080i) is the next step up (and then, 4K). If it says 720p, then you already have HD I guess. As of a couple years ago broadcasters used either resolution -- depends on the network first, and then the provider (Comcast). I have Fios, and actually am not sure what they broadcast these days. All I know is, I have a 4K TV, and cable doesn't broadcast that except on perhaps a couple dedicated channels for specific events (and even then, you need their latest box). (As opposed to some streaming options that do broadcast on 4K more often -- for instance, most new Netflix series are in 4K).

For smaller screens, resolution doesn't matter as much. But if you have a big screen TV, you'll want a higher resolution. 4K looks incredible! But it's still a novelty in most cases. But in the end, if it looks good to you, that's what's important. If you have $ to burn, when your TV goes, upgrade....