That seems to be a common question having looked into render farm solutions myself. It all seems to stem from the power bill each month and whether it bothers you or not financially. I look at it this way, if you get enough midrange computers to render your scenes and animations then you can get more done in larger chunks but if you use less computers with higher end hardware then you’ll get the same amount done but using less computers. There’s probably a balancing act factor as well in that you may get farther with the midrange computers but you have to consider how many it will actually take to outperform the higher end machines without dedicating half of your house/office to render computers and, again, how that will affect your electric bill in the long run. I’m hoping for somebody to test out those Xeon Phi cards to see if they’re any good for 3D rendering or not, if they are then it might make individual and small business render farms a lot easier to handle.
EDIT For stills I think it would be best to use the higher end hardware as you would be previewing the render as well. For animation though it’s up to preference and work situation as stated above.