OneWorld
The choice is yours - If you're happy to open your wallet and spring for Microsoft Office for Mac, you won't regret it - it is a high-quality piece of software, in some ways superior to its Windows counterpart and totally interoperable with it - I know this because i work between both versions every day.
The Open Source Office clones I haven't tried myself, but hear good reports from those that use them on Windows (I don't personally know any Mac users who use them.) I believe the Mac version is a Java app (open to correction on this?!) and I'm not too fond of Java personally, I find it clunky and buggy, especially on the Mac (apart from Azureus, which seems quite stable).
I can't think of any reason (apart from perhaps certain games and perhaps a few specialised finance apps) that you would want or need to boot into Windows on your Mac
