Macs give you confidence that when things go wrong developing a Rails application that it is your fault because almost all the library developers are using them so they test using them and Windows testing comes second if at all.
On a Windows machine there are always two possibilities - you screwed up or there is something broken for Windows. It is a PITA to deal with library dependencies where one of the chain is broken for Windows and you don't find until after installing other things and screwing up your whole setup.
Eliminating the second possibility every time is worth spending some money on because it will save you time.
