Do you use Office primarily for personal stuff? Or is it tied into work (ie: use Outlook cal/scheduling w/ other people a lot, etc). If it's mainly for you, go with the Mac version. The file formats are [finally] all fully compatible regardless of platform so any documents you need to exchange with others will be fine. Using Mac software on a Mac platform should result in fewer issues.
However, if you use Office primarily for the calendar / scheduling stuff with lots of people (Exchange servers and such), the overhead of Parallels + XP w/ a Win version of Office may be the way to go.
If you are mainly looking for a good office suite (word processor, spreadsheet, database, etc) that is compatible with Microsoft Office, check out
http://www.libreoffice.org/
Since it's free, give it a shot with any documents you may already have.. see how it works for you.
(I run it both under Mac and Para/Win)
Last edited: Jan 31, 2011