The Steeda four link is awesome!

Steeda is Great! and makes a great Spring with a decent rate and great ride.

MM is has a few very effective set-ups as well.

MM is more Aggressive with Spec H&R's. As well the ride height is lower then Steeda.

Its a very open question! Steeda will be a few dollars less however it clearly depends on what you are trying to achieve from your Stang..

don't forget your Brakes either
 

If you're corner carving, the stock GT brakes won't do that well, depending on the Track.
An excellent upgrade is a kit from any Ford Dealer that upgrades your brakes to 13" Cobra rotors & calipers. An expert on this is Ed Nicholson at Fines Ford in Toronto.
If thats too pricey you can always upgrade what you already have with a slotted rotor & Hawk Brake Pads ( www.muggziperformance.com )
Rotors are around $136.00/pr for the front.

Mach1steve
Pic of me on the BIG Track at Mosport !