OK here's my take:
If you have a new (2009 or 2010 model) Macbook or MBP, go 7. However, driver support isn't great on the older ones - maybe it's improved? I had issued with the RC on my wife's white Macbook (early 2008).
Now, I KNOW that I'll hear a lot of crap for what I'm about to say, but... I would pick Vista over XP on a Mac. I have copies of both XP and Vista (but not 7), and Vista runs FAR BETTER on my 13" MBP (the brand new 2010 model) than XP. Can't really explain it, and believe me, it pains me to say it, but, at least for my MBP, I'd pick Vista over XP.
However, if you're having to buy a new copy, don't get anything but 7. 🙂
It just seems like driver compatibility with Vista is better...