Use the 1/2" cbu...on a wall, the 1/4" stuff is too thin and would not support the tile well. It works fine on a floor, though. The cbu (cemtaeous backer unit) is not structural. Also note, that it is not designed to be used on walls if the stud spacing exceeds 16".
When you get ready to tile, make sure that you use the alkaline resistant fiberglass tape (looks like the stuff you can use on wall board, but it is not). You can put this on as you tile, rather than doing it beforehand. If you do it this way, it is less likely to dry with a hump that makes it hard to keep the tile level. roofing felt is okay, plastic may be cheaper and easier. Run it down over the tub tile flange (if it has one). Then run the cbu down to about 1/4" from the top of the tub surface. Fill this gap after tiling with caulk. It is best to also caulk the gaps at the corners instead of grouting. It may be useful to shim the studs out so that you can run the cbu over the tile flange, otherwise, you'll end up with a flare at the edge by th etub. My unprofessional opinion. Check out www.johnbridge.com for some expert advice.