Sounds like you'd be moving the same documents back and forth a lot, so Docs 2 Go, or QuickOffice would be your best bet.
The iWorks apps are great for creating documents on the iPad and have good export/import compatibility with Office documents. But the other two apps are more compatible, and since every time you pass an app back and forth you take a chance creating formatting errors, better stick with the most compatible app you can get.