Hi tim1198,
There is more information about mattress encasements in post #2 here but if your goal is to provide waterproof protection to the mattress and not “water resistant” protection or protection against dust mites or bed bugs then I would use one of the membrane type encasements.
All of them are very similar and use similar membranes and the choice between them is more about different pricing policies and the type of fabrics that are used in the protector. While there are different types of membranes … none of the manufacturers provide specific information about how their membranes are manufactured so there is no way to make a “scientific” decision based on the durability of the membrane itself (see post #2 here). Every one of these types of mattress protectors have some complaints about the membrane “failing” but I suspect that in most cases this would be because of exposure to excess heat and not following the proper care instructions but in the case of a 6 sided encasement that was around the mattress on a more “semi permanent” basis (vs a 5 sided protector) and that also had a mattress pad on top of it these types of issues would be much less common.
If you are also concerned about protecting your mattress against bed bugs then I would choose an encasement with a suitable locking zipper that has been tested against bedbugs and in this case the Protect-A-Bed encasements would make a good choice.
Phoenix