Both are great programs. If you're concerned about price, Open Office is your best bet. If you want speed and performance, MS Office is your best bet (yada, yada, multi-thread processing / cpu architecture compatibility and extension handling).
For compatibility, I like Open Office and it's open-friendly structure. Best overall solution (assuming you have the money for Office) is BOTH! Why not have both on your machine? Open Office's super small footprint won't hinder your machine if added to MS Office.
If I had to make a sure choice and money is no option, MS Office all the way. I do suggest, however, staying a release behind. I'm using 2k7 atm, and it serves me well. That will save you a few dollars.