Eibach's pro kit will give you about 1.5" all around. Some people have said that the drop didn't give them 1.5", but I think they didn't wait until the springs settled.
The H&R's will drop you a little more than the pro kit and are more performance oriented. They will be stiffer making the ride a little more edgy, but the extra drop is worth it.
If you are concerned about scraping, I'd go with the pro kit and leave the iso's in. You'll still get a noticeable drop compared to stock...your fender gap is probably near 4" right now. The pro kit will cut that damn near in half.
But if you want the most drop with good springs, get the H&R's. I'd still leave the iso's in with those though. (Don't bother with those ebay springs claiming a 3" drop, they are garbage)
Brian (Jimmy_Beaner) has the pro kit on his GT...I think it sits perfect.
Here's a Mach 1 with the H&R's.
As said the spring rates are important...if you keep your stock shocks/struts you don't want the rate to vary too much, or else you'll wear your shocks/struts out real quick. You will want cast and camber plates if you do a drop 1.5" or more, or else you'll wear your tires unevenly. Maximum Motorsports CC plates are the best...heard nothing but great things about them.
Keep in mind you can order this stuff through John at NQS Performance (site sponsor)...PM and him and he'll get back to you.
After wheels, a drop is the next best thing appearance wise...getting rid of that stock 4x4 look really changes the looks of your mustang. Either way you go you'll get a noticeable drop. You may want to consider shocks/struts as well since you'll be tearing that area apart, plus they'll be rated to handle the more aggressive springs. Better to replace everything at the same time to prevent problems farther down the road.
Let us know what you decide