I agree that the primary reason to use a Mac for Rails development is TextMate. TextMate offers you numerous shortcuts for common Rails code, e.g. typing bf + tab to get a before_filter. Also, much of the work you do in Mac Terminal is easily translated to the Linux command line, which will definitely be an asset as you expand your architecture credentials. In addition, Rails utilizes many open-source technologies and Macs help expedite the install process by making compiling and building these applications fairly straightforward.
