You actually have lots of good choices for springs. But you really don't wont to go any lower then 1.25". Mustangs do a good impression of being a lot higher then they really are.
Steeda Sports or Eibachs are both good quality springs that will lower your car the right amount. I've gone though all sorts of suspension combos, and basically both will feel stiffer then stock, but not uncomfortable by any means. You have to be careful around speed bumps and similar, but with a 1.25" you will be able to clear most urban obstacles with out much trouble.