all depends on how comfortable you are with computers, really. if you know what the registry is and are comfortable tinkering with your system, a windows machine is probably the cheapest option. if you'd rather everything “just works” you're probably better off with an apple – they tend to be more stable due to a smaller pool of hardware/software options available, plus the lack of internet exploder.
it also may depend on what software you want to use – i'm pretty sure logic is still only on os x, for instance.
