I'd go mac, as you can always install windows on it. I had a pc since 1995 and went over to mac in 2006 as my pc was on its last legs and i needed a laptop as i was off to China for an extended period.
Loved it once I got it, much easier to use and so much less hassle and fiddling. Its true that you can get better hardware for the same money with a PC, but unless you plan on playing the latest games it really makes little difference.
I would say though, talking about specs, if you can afford it get as much RAM in there as you can. When I got mine you couldn't get more than 2GB inside it, they now take 4GB. My girlfriend bought her iMac with the basic spec (512k) 3 years ago i think, and its really struggling now. If you want to save money don't bother upgrading the Hard Disk, as the chances are you'll fill it with millions of raw files anyway - I've got a big 1TB on my ethernet and keep most of my stuff backed up on there.
I'd recommend extended warranty aswell, more for peace of mind. They dont break often, but when they do it can be expensive, and if you're out of warranty a pain. Others may say not to bother though, personal preference i guess.