Office hands down. If you're doing word processing or spreadsheets, nothing beats office. If you have a few dollars extra, I'd suggeset grabbing Keynote from the App store as well though, it's great for presentations.
I'd suggest Office because you'll probably be needing to share your files with others. iWork can convert but the results are sometimes not the greatest. Don't buy office from a store- you can probably get it from your school for cheap.
Based on the above sentence it probably won't matter which dictionary is better.
Just go with Office. In the educational/business environment it's the standard, for better or for worse.