Hi ceasar2k6,
In general … latex is the gold standard for durability and is the most durable of all the foam materials (polyfoam, memory foam, and latex foam). There is more about the many variables that can affect durability and the useful life of a mattress in post #4 here and the posts it links to. A cover can have a significant effect on the feel and performance of a mattress but in most cases it will have little effect on its durability (unless it includes layers of low quality quilting foam or fibers that are too thick in which case durability would be reduced).
Memory foam is too soft to be very supportive and is only used in comfort or transition layers where it can help with contouring … not in support layers. There is more about primary support, secondary support, and their relationship to pressure relief in post #4 here.
Unfortunately … there are too many unknowns, variables, and personal preferences involved for anyone to make specific suggestions for someone else based on specs (either yours or a mattress) or "theory at a distance (see mattress firmness/comfort levels in post #2 here).
The list I linked earlier includes the members here that sell latex mattresses online but you would need to decide on the “value” of each of them based on the guidelines in post #13 here and the parts of your personal value equation (including price) that are most important to you. Lower cost can sometimes mean a mattress that is either less suitable or less durable (depending on the design or the type and quality of the materials in the mattress).
Phoenix