Would like to try Windows Phone. Can you help?

Hi guys,

Noob here. I'm in the position of needing a stop-gap phone for about 4 months and I am considering giving Windows Phone a try. I have a few questions and wondered if you guys can help.

1. I listen to audio over bluetooth in my car a lot. Has anyone had problems with this on Windows Phone?

2. My biggest social media needs are facebook, twitter and SMS. Any problems/suggestions there?

3. I'm looking at either a Lumia 900 or a Lumia 620. Their prices are similar, but one runs 7.8 and the other is on 8. How significant is this difference in terms of functionality? Are there any other phones I should consider? (I'm a bit of a Nokia fan who still occasionally pulls out his e71.)

Thanks for your help.