I'm using both a mac and pc laptop at the mo (one's a work one) and I have to say that the 4-5 year old Mac Powerbook G4 still knocks spots off the brand new Compaq Presario in terms of quality of the hardware (screen, keyboard etc), general performance and ease of use. Only (major) downside is the price, both of the hardware and software I guess!
There are also compatibility issues to take into account (do you share lots of documents with users who use certain software packages)? Although most Microsoft documents (Word, Excel) are in theory compatible you do get the odd blip where PC users using older Microsoft software can't open the new Mac Microsoft files. I'm not really up on this though, worth checking with experts if you can find any!