Mac. Lots of better features for being a web developer versus a windows. Being unix-based, you get a lot less headaches when dealing with Ruby or any other language. Textmate is also my favorite applications.
Other cool features about a Mac:
- Less headaches, it's very stable.
- MobileMe syncing is great. Set it and forget it, plus it's not that much if you get a few friends to help you throw on a "family" plan.
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Transmit Disk is an awesome feature to have for backup with S3.
Note: I have a PC as well, and I find myself using my little 13" MacBook Pro more than the PC. Preference is preference, but I feel much more productive on a Mac versus a PC.