I've used Seachem and salifert to test cupramine, salifert is easier to read and seems more accurate (based on how the test is performed vs Seachem), but I usually use both kits and compare to make sure my reading is accurate. With Seachem, I also always run the reference test for comparison.
I am growing quite wary of the cupramine method. Too little it is not going to be effective and little too much it will be harmful to the fish. For the next round of QT, I am thinking of going the Hypo way.