I had FRPP C springs on my old 01 GT. The handling and ride were great but the car just didn't sit low enough for my liking.
There's a pretty big difference between the FRPP springs and H&R springs in terms of how much they lower the car. H&R offers a pretty drastic drop while the FRPP's are only slightly lower than stock.
Personally, I think that 99.9% of mustang owners would be happier with the H&R drop.