Unless you're burning it on BluRay, your DVD is SD. To keep file-sizes down, I've exported to 480p (because that's what it's gonna be on the DVD), but when I did that, I ended up with letterboxes on ALL 4 sides. So, now I export to h.264, 720p, and then I let my DVD-burning software lower the resolution.
For youtube and vimeo, I personally prefer h.264, 720p. In my opinion, I don't really see the need for full HD for something that's going on the internet. Of course you could do h.264 at 1080p, as well.