I did have Spoon , gen 1 is a bit lower than gen 2 (I had one of each), but gen 1 only locks on one side while gen 2 locks on both sides which is imho a lot better. Haven't used any other brand so can't comment!
I bought my 9 with drivers spoon low slider (gen 1) and then got gen 2 (as gen 1 was discontinued) passengers from spoon sports europe. Both lower the seat quite a bit, but exactly by how much I could not tell you. I no longer have them as I am using side mounted seats now so can't measure them either sorry.
i've a HKS kansai lower rail for my recaro, A lot better position over oem plus the lock is on both rails unlike the spoon which only locks on one side.
I have a pair of spoon gen 1 rails in my ek9. They do only lock on 1 side but I don't find any problems with them plus I think the position of them is spot on