if you mount it in the front, then you are pulling cool air from outside the case to the radiator which is good. the bad news is that you are now pouring "warmer" air into the rest of the case.
if you mount it in the back, the radiator will be cooled with the "warmer" air from inside the case, but you will help with overall case cooling by pushing all that air out of the case.
i have mine mounted rear just becuase its the best stop in my case.