The main reason I guess is that most of my servers run linux and I don't need to mentally make the switch from one flavour of unix to another ten times per day. Stuff on my dev box lives in exactly the same places as it does on my production machines.
The mac is very slick though, but that's not a good enough reason to buy one if you have to choose between them.
(I do most of my development at home on a Mac. I don't like dicking around with my operating system though.)
The Mac OS is prettier. I would suspect that it gives you a better experience on a laptop than Ubuntu does. For some reason, I like my macbook better than I liked my thinkpad before that. If you don't care about such things, you might be better off with Ubuntu.
The mac is really pretty, but almost all the hard work is done in a hp laptop connected to a 25.5" monitor, external mouse and keyboard and running windows xp and a couple of virtualbox machines.
Use whatever tool make you more productive. Don't waste money in a mac just because fashion.
What do you do?
I have a Mac right now. Why? I do tons of web dev and I need to test multiple environments- mac, windows etc. I can do all that with one machine via virtual machines. Not so easy to do mac os testing with a PC.
I do most of my dev in a debian vm that is setup to exactly match our production machines.
The problem that I am running into is that Rails behave differently on my Vista laptop than it does on my VPS and I and it's causing me quite a bit of grief.
I just want to be up and running as soon as possible.