I'd deffo go front mount if you want to change it anyway.. they are sooo cheap now too for the Jap stuff.

I've got a front mount on my GTi-R (as you may know the std cars have a large top mounted cooler, literally right over the engine!)

A lot of the GTi-R boys like the uprated ARC top mounted coolers but I really don't see the point. They always cry LAG! but I've had a few GTi-R's and with the front mount fitted it's hardly noticeable, it far outweighs the negative effects of heat soak and wooly performance after a few mins of spirited driving and stops.

The Impreza's top mount isn't in too bad a location compared to the GTi-R but it's still not ideal. If you're going to look for an uprated cooler then go for a front mount. The point Will raised about high speed and missing the intercooler is a valid point too.