Microsoft Office because it's the program I've used the most out of those two and therefore gotten more used to. Second, Open Office is just horrible on my computer, everything is out of place.
Open Office if the GUI was nicer.
I prefer Open Office because it is free, but I think MS. Office is better to use if you are sending files to people like your boss since I notice when I make files in Open Office and open them in MS. Office the formating looks different.