Depends on how low do you want to go? If you get springs Eibach is going to be the better if not best route to go. Eibach has good struts/shocks but if you want the best go with Koni. The Eibach sportline lower your car 1.60 in the front and 2.0 in the rear. And the Eibach Pro-kit Springs lower 1.3 in front and 1.4 in the rear. Most important thing get the Ford Racing GT500 Style Strut Mount Upgrade they are the rubber strut mount that goes on top of the new spring and strut. This keeps your new springs from making a clunking sound that most mustangs get after being lowered. For my 12 v6 the eibach only go 1.1 in front and 1.7 in the rear so i decided that I needed coilovers to get the stance that I want on my stang.
This is the link for the rubber strut mounts I am talking about
Ford Racing Mustang GT500 Style Strut Mount Upgrade (05-13 All) M-18183-C - Free Shipping!
AirRaid CAI, BBK Ceramic LT Headers, BBK Catted X Pipe, Borla ATAK, MPT 93/97 Tune, Steeda LCA's and UCA, Steeda Relocation Brackets, 373 gears, WaveTrac LSD, Dynotech Driveshaft. Prothane Motor Mounts, Whiteline adjustable panhard bar & brace.Ground Control Coilovers with Koni's custom valved. Coming soon Circle D 3800 Stall and Lone Wolf Intake Manifold.