AFAIK - Spotify's system keeps track of streaming sources, and won't let you stream on 2 devices simultaneously. That doesn't mean you couldn't use your subscription on 2 devices... it would be similar to sharing your children post divorce..
e.g. Wife listens to Spotify on way to work... you listen on the way home. (or similar.)
Spotify does have a private credentials login, so you don't have to authenticate it with FaceBook, if you don't want to.
IMO - ZunePass is a better deal, since you can download tracks to your computer, stream on multiple sources simultaneously (e.g. WP7, Xbox, and your PC)... that said, you have an iPhone in the mix, and there's no Zune client for iPhone. Downloads from Zune Marketplace won't play on the iPhone, since they are DRM'ed. (unless you buy your tracks)
Hope that helps!