If you can, you'd render at 1080p. Because your offering the best quality (visual) content to your viewers.
Viewers having the option to watch in 1080p will make it look like you've put more effort into your video, putting a lot more time sitting through longer render, compression, and upload times. But 720p looks more or less the same - unless you have a really big HD TV.
But for anyone reading, I'd go with what's best for your internet speed.
Personally, I render all of my videos in 1920x1080 comp at 29.97fps (movie fps). This is because there's no need for 60fps in a video.