I haven't used a compstock but I do have an A3 recoil pad on my Remington 870. The thing is very good- probably cuts recoil by about 30% or so if I was to tag a number to it. The only complaint anyone ever had is my brother did not like the "squishiness" as the pad warped a bit to one side with his shooting posture. On the other hand, he can "micky mouse" slugs at 20 metes (three in a group, two high on each side like "ears"). I got mine for $30 from Brownells, mainly because I wanted to keep the original walnut buttstock.
The Compstock is more expensive ($80-100) but from everything people have said it is even more effective than the R3 (50%+ recoil reduction from what I understand), and many guys will not ever go back to a "normal" stock once they have a Compstock installed. Knoxx also has a rep on the baord and they have great customer service from everything I have heard.
IMHO, it is mailny do you want to be able to keep a nice wood stock and a little less expensive, or go all out and get the best.
I am sure Biggerhammer will chime in here, he loves the Compstock and can give you a lot of good reasons to go with it.