No, you need two layers of ply with the seams offset (may not need to offset if you can do it in one sheet for each layer). One sheet is unlikely to handle someone standing on the counter (a child, or maybe you!). If you have a counter on the cabinets now, it's likely it is made of particle board, and you definately don't want that underneath tile. The membrane is easier to install, lighter, thinner than cbu, so it is my preference. You can use cbu, but I'd want to use something like RedGard on it to help with keeping everything dry under there, especially around a sink.