Well, for the price, you can't really beat the king cobra/heavy duty on a stock mustang. When you get serious about hp, step up to a better clutch, or use this one till it gives. I've had mine for years now, so no doubt about the longevity. It's a decent clutch, truthefully, after installing a spec on one of my brothers mustangs, and a centerforce dual friction on his other, I instantly became spoiled, as the king cobra sucks major ass compared to either of those clutches. However, you definetly pay a premium for them too, as my bro paid I think near $350 or so for the centerforce.
Really, the choice between the heavy duty and the king cobra is a simple one. Remember, no diff in performance.
I'm a big dude, and I love the stiffer clutch pedal, +, it's so stiff that people don't like to drive my car, which is fine by me. If you love stiff clutches, get the heavy duty. If you do a whole lot of city commuting, or don't have quite the left leg, get the king cobra.