This is just my opinion, but having both loaded onto my iMac (iWork '09 and Office for Mac: 200
I find I like each for different things.
For example, I prefer Pages for simple letter writing or creating specialty documents (like brochures, cards, etc.), but I always turn to Word for heavy-duty work processing capabilities, in particular mail merge.
Number is fine, but coming from an accounting background there is no replacement for Excel. It is the best spreadsheet program out there. Period. If you need that sort of thing I can't recommend Excel too highly.
Ultimately I'm certain you can work with either iWork or Office. Or the various free options like Google Docs, OpenOffice or NeoOffice.
Oh, one thing I do really like about Pages is how you can Save As documents as PDFs. I have found this to be very handy when I know I'm going to need to share a document with another party. They do not have to have installed anything other than Adobe Reader.