Based on what you are trying to do, I would say one good quality sub is better than two cheap ones.
Two subs are clearly better than one, only if you were able to run stereo subs (each sub playing a separate channel). You could run stereo subs, but you would have to run them off the speaker terminals, with a sub that can handle such a connection (high pass). You would lose the separate sub channel on your HT setup. Such a setup is great for music.