My choice here would be hypo as it forces you to run both the display and QT for the 6 weeks needed but also fell hypo is easier on the fish, high copper levels will be just as fatal where SG a little higher or lower will have little to no effect on the fish.
True, but a SG a little higher than it should be can also allow the parasite's free-swimming form to survive. I've also read in a few places that there're some hyposalinity-resistant strains of ich going around, so you can't necessarily count on hyposalinity to work all the time. If it doesn't work, of course, then you'd need to use copper.