There are pros and cons to both.
With the top mount, there is less piping, therefore less restriction, therefore easier to blow air into the engine combustion chamber. However, the cons are that there is an awful lot of heat soaking into the intercooler from the hot engine, so if you are sitting in traffic, or running the car for an extended period of time, the intercooler might become less efficient. The other problem is that although the underhood ducting is cool, it doesn't have as much air moving through the intercooler as you could if you put it on the front.
The front mount has more piping, and as a result, more restriction. However it is not going to have as much of a problem with heatsoak, and is more efficient (generally speaking) than a top mount is because of how much air is flowing over it.
So which one is right for you? Front mounts are probably better if you are thinking about putting a bigger turbo in so you can overcome the restriction through brute force, and if you plan on running your car at the track for extended periods of time.
Top mounts are better if you are looking for an easy way to bump up horsepower.
Think of it this way (speaking generally here, this may not be the case with the specific intercooler products available for the speed3)
The top mount will give you better gains out of the box.
The front mount has potential to give larger gains when proper tuning and bigger turbos are used.
The top mount will give bigger gains before heat soaks into it, but after it heats up may not give you much extra power at all.
The front mount will not get as much heat soaking into it and so will continuously give you that smaller gain.
If any of my information is incorrect please let me know, but this is how I understand the system to work.
