Hi brooksdj7,
Welcome … and I’m glad you found us 
Just in case you haven’t read it yet … the first place I would start is the tutorial post here which has all the basic information, steps, and guidelines that can help you make the best possible choices … and know how and why to avoid the worst ones. Most of your questions will be answered there.
The sagging in the middle issue is from the use of lower quality foams in the comfort layers of a mattress (which are the weak link of a mattress) not from the type of foam used. While all latex foam would be a durable material (although there are different types as you can see in post #6 here) … there are many different densities and grades of both polyfoam and memory foam and lower quality/density versions of both will soften and compress much more quickly (sometimes in just a few months) than higher quality/density versions with both polyfoam and memory foam. You can see some of the density guidelines I would suggest in post #4 here. There are some innerspring mattresses that use very high quality polyfoam in the comfort layers that can last for a very long time (especially if they are two sided).
While the type of mattress or material that is best for you is really a preference choice … as long as you make sure that any mattress you are considering and that is a good match for you in terms of PPP uses good quality and durable materials (in the comfort layers especially) and if you follow the steps in the tutorial post one by one you will have the very high odds of making a great choice … no matter which type of mattress or materials you prefer.
Phoenix