there is no gopro that's just 720. you either get the HD Hero, which has 1080p30p, 960p30p, 720p60p, 720p30p, and standard def, or you get the HD Hero 960, which just has 960p30p, 720p30p, and standard def.
battery life should be the same on both as far as i know.
quality on the normal HD Hero is obviously better because it has the 1080p instead of just 960p. It also has 720p60p which is nice because 60fps is important to a lot of people, especially if you'll be filming others and think you might be doing slow motion. Plus, 960p is really only handy for POV footage. It's perfect for POV because it's 'tall,' but I'm not a fan because it's not widescreen.
If you can afford the normal HD Hero, there's no reason you shouldn't get it. The 960p has no advantages over the 1080p other than the fact that it's cheaper.