You can create a great bumper with a sheet of CDX plywood, no question.
Quality AC sanded pine plywood from a reputable mill is a good alternative. The price difference between that and a sheet of C3 birch plywood like
this one has been small enough that I usually take the birch plywood, which is nicer to work with. For example, the Menards web site is showing 3/4" thin veneer birch plywood at a lower price than a 3/4" AC sanded pine sheet -- both about $40. That's enough plywood for two sets of bumpers, with plenty to spare. Of course, we'd have to see whether that pricing held up when you got to the store, but you get the idea.
I've also had issues with mediocre glue in lower grades of plywood from the home center. Finally, the thinner veneer holds staples better.