Is there a substantial difference between using a Mac or a Windows machine for web development (particularly RoR)?
No matter if you are on Mac or Windows you should be using a Linux virtual machine for development, ideally running the same operating system as you deployment environment (I like Ubuntu for this). Vagrant is a popular tool for managing this kind of setup.
That said, for web development you should be on a Mac, with Windows running in a virtual machine. You need the Mac in order to test in desktop Safari and use the iOS simulator to test in Mobile Safari on iPhone and iPad.
I don’t think it is possible to run OS X in a VM on Windows without using cracked versions of the installation disks, and you have no guarantees that technique you use will continue working for later OS versions.