Depends on a few things. Putting it in an exhaust configuration on the top of the case means you eliminate most of the heat energy from your CPU from the inside of your case, but you have more noise since you're opening the top of the case up in order to do so. Also, you would have to be a bit careful to maintain positive pressure inside your case if dust management is a priority, unless you remove the back exhaust and use enough intake fans up front with enough air flow to compensate.
Mounting it up top also may or may not work if you have large memory modules, so you'd have to verify that's an option.
If you mount it up front, you have the heat from the CPU blowing into the case, but if that turns out to be the easiest solution...