I've only ever had Eibachs. Not by choice, but every Mustang that I buy seems to have them on and I think that they are okay. I've heard great things about the H&Rs though. I would love to try some out as every review I have heard has been positive.
I've only had experience with the Eibach's even on my coilover's. They've done well for me. I've heard good feedback about H&R's just never had any experience with them.
Aftermarket Spring Rates
Manufacturer/Part Info Front Spring Rate (lb/in) Rear Spring Rate (lb/in)
Ford Racing B Springs (M5300B) 425-530 (progressive) 200-300 (progressive, solid axle)
Ford Racing C Springs (M5300C) 650 (linear) 200-300 (progressive, solid axle)
Ford Racing F Springs (M5300F) 460-570 (progressive) 170-310 (progressive, solid axle)
Ford Racing G Springs (M5300G) 500-570 (progressive) 170-310 (progressive, solid axle)
Eibach Pro-Kit Springs (94-04 Mustangs) 425-530 (progressive) 200-300 (progressive, solid axle)
Eibach Sportline Springs (79-04 Mustangs) 425-630 (progressive) 140-295 (progressive, solid axle)
H&R Sport Springs (94-04 Mustangs) 490-575 (progressive) 205-250 (progressive, solid axle)
H&R Sport Springs (99-04 Cobra) 700-760 (progressive) 685 (linear, IRS)
H&R Super Sport Springs (94-04 Mustangs) 700-760 (progressive) 275-300 (progressive, solid axle)
H&R Race Springs (79-04 Mustangs) 750-850 (progressive) 260-280 (progressive, solid axle)
H&R Race Springs (99-04 Cobra) 750-850 (progressive) 770 (linear, IRS)
Steeda Sport Springs (79-04 Mustangs) 650 (linear) 200-250 (progressive, solid axle)
Steeda Sport Springs (03-04 Cobra) 720-780 (progressive) 650-790 (progressive, IRS)
Steeda Competition Springs (79-04 Mustangs) 750-850 (progressive) 250 (linear, solid axle)
StangSuspension Springs (94-04 Mustangs) 490-575 (progressive) 205-250 (progressive, solid axle)
I've got the Steeda Sports on mine. They handle great on the street and are reasonably comfortable. A lot of people run the Eibach pro-kits. Too soft of a spring in my opinion. Steeda has used tokico shock/struts with proven results.