eh, stress coat is decent... but not as a water conditioner (well it is, but it is way not economical). I add it to all my 1)Mothers tending fry 2)fish being moved 3)fish in quarantine, hospital, etc. Helps keep their mucus coat intact.
Stress zyme is, excuse my language, crap. About as good as Cycle made by hagen. It may have some nitrifying bacteria (nitrobacter, nitrosomona, etc) but doesn't contain the key family of nitrifying bacteria for the aquarium, nitrospiros. I believe there is only 1 product on the market that can offer this and that is "Bio-Spira". Go to marineland's website and look at the history going into creating Bio-Spira, it is worth the high price.
I used to use API's water conditioner, and it does work nice (I keep some around for emergencies) but I either use prime or the pond water conditioner from petsmart. The latter is so concentrated (plus the tiny amount of tap I use (only about 10% of all my water changes)) I don't really need a lot and it lasts forever.
If you are going to pick a brand and follow it religiously, I would go with SeaChem and Marineland. I have had no problems with ANY SeaChem product I have used and there is only 1 or 2 that I would be hesitant of using.
By the way... this is not an advertisement.