It just depends on whether or not you want to sacrifice handling ability on the street for quicker launches at the track. It's simply a matter of which is sounds better to you: 1) the car feeling like it's on rails but having some traction problems coming out of the hole... or 2) being able to squat and leave the line like a bat out of hell but having to take it somewhat easy in the curves.
I personally spend way more time on the street than I do at the track, so I chose to go with a stiffer suspension. That's just my personal preference though.
Also, I recommend you check out Maximum Motorsports
, they are the best place to go for Mustang suspension parts, even though they most cost a little more than some other places. Their parts are top-notch.