Why not both? Install one of them as the base, grab something like VirtualBox[1], and install both of them in virtual machines. Take a week or so, then pick your favorite. As a bonus, once you've picked, you can mimic having a production server on that machine with a Linux VM with just the basics (Apache/lighttpd + interpreter).

I'm actually in a similar boat. I'll be building a dual-core box (intel barebones + cheap processor + 4 GB for <$300) to use as a dev machine in addition to my Mac.

[1] = VirtualBox doesn't support multiprocessors for hosts. You may want another solution, but I haven't investigated the free Linux virtualization market at all (I think VMWare has a free Linux server, but I have no idea of its features). Paravirtualization is another option for a Linux-on-Linux solution, but I think it's too complex for this.