Well it all depends on how serious and how deep are your pockets.
If you have some serious cash to spend go with a GR40 setup and you'll love your Mustang like never before.
If you are more on a budget the price on Roush Trak Pak suspension setup has come down significantly and is now a lot more affordable then the GR40.
Finally also consider what you'll be using your car for. Is it an all out racecar then you're probably going to want a full adjustable setup with more stiff heim joints for bushings. If you'll be driving your car on the street you may find a good boxed setup your best bet and perhaps add in a few tweaks here and there just to custom taylor the setup to your preference and needs.
I needed a car that was still drivable on the street as well as being able to tackle the Auto-X and twisties so I went with a Roush stage 2 setup, added some J&M
LCAs and bolted on a watts link for the added precision. Setup cost me less then two grand and I think it was perhaps the best money spent on the car.