I have had the B springs in my 79 5.0 Capri since 1992 or so. I love them. They did lower it more than I intended, but I have grown to really like the look and love the handling. They are good springs.
I think the C's were even higher rate, for more handling, etc. Check the FRPP site or catalog for their description.
I have them in my 2.3 notch. They are pretty easy to install since they are fixed rate. They didnt really drop the front much if at all, but with the GT sway bars it really stiffened it up ant took care of the wet noodle feel the car had. I would probably cut half a coil if I were to do it again. One thing I noticed that I never had with any other aftermarket spring is that for the first month or so, these things were HARSH! They have mellowed out quite a bit now and ride pretty decent. They are probably a little rougher than they would be in a V8 car but for the price, you can't beat em.
Cheaper...yes, but better would be a matter of opinion. I have Eibach pro-kit springs on my 90 5.0 hatch. They were an absolute bitch to install but they are the best springs in terms of performance without sacrificing ride comfort that I have ever used, hands down.
If you like rocks, the c springs will be fine. They are a very high spring rate for the lighter 2.3. In terms of handling, the C-springs will do better than the eibachs. This is simply because the less suspension movement you can get in the front, the better the camber will be. In the mustang suspension, you actaully gain positive camber in suspension compression. It is the opposite of what is ideal for handling.