they are both good...
I use Office for all of my word processing and spreadsheets documents, it's great. I just graduated from high school and recently purchased a 15'' PowerBook that also came with the 30 day trial of iWork. I don't use pages much because I don't make too many flyers or anything, but before I graduated I had the opportunity to use Keynote, and Keynote blows PowerPoint away. If you think that you might be doing a lot of presentations or slideshows with your computer, I would definitely get iWork just so you could have keynote.
But if you can only really have one, go with Office. Work and Excel are great applications (as other posts have said) and PowerPoint is good enough that you could do some decent presentations. But after using Keynote for a few last school projects and checking out a few things that Pages was capable of, I went ahead and purchased iWork with the education discount.
And oh yeah, I hate windows as well, but you gotta remember how compatable Word and Excel are.