Ride quality is dependent on what you like. If you want a softer ride, obviously you go with a lower spring rate.
Hyperco, TRW and VBP all make good springs. Each have a different theory on how to build a spring. Hyperco uses a high arch spring with low spring rates. The Hyperco EZ Ride spring is 200#'s which is equivilant to a stock 9 leaf. The next spring rate up is a 330# spring.
TRW uses the same approach as Hyperco since they both use GM molds for their springs. TRW just doesn't go as low as the Hyperco in spring rate. I think the lowest rate is 315#'s. That is what I have on my C3 and it's a smooth ride.
VBP uses a low arch spring with a higher spring rate but you have to use the 8" spring bolts no matter what. Not that it's a bad thing.
With different spring rates also comes different shocks. KYB GR2's or Bilsteins work best. When you choose your spring rate, the company you buy them from should be able to tell you what shock will work best with that spring. The 2 different shocks have different rebound rates and the nose pressure rates and they should be matched to the spring.
Couple other things to watch for with a fiberglass spring. Keep heat off the spring. Exhaust heat can cause premature failure. Normally header wrap around the spring and/or around the exhaust will help the heat issue out.
Another thing is do not use any acidic based cleaners to clean the spring. This will and can cause the spring to split. Just use a wet rag if you have to keep it looking clean.