Hey Vanessa, you could also say "Which side do you take?"

It's fairly common. But depending on context of the situation - it can be confrontational. Alternatives can be 'who are you backing?' or 'who do you support?'. Again all depends on if this is in a sport setting, political party etc.

You could also say "What (or "which one") are you leaning towards?" But yours is totally normal too.

Yes! It's quite appropriate for an informal situation. :)