If you export to Word format in iWork the file can be opened on a PC.
It's can be simple, will you exchange a lot of Office documents with other people, like many times ever day? Then pick Office. Once in a while to open and save Office files in iWork is okay, if you do it a lot it becomes a pain. Do you need 100% file compatibility with Office? If so, pick Office, iWork does a good job but not always a 100% perfect job.