Hi bhindiana,
If you have a mattress that is still in good condition and doesn’t have any soft spots or visible impressions and the only issue with the mattress is that it’s too firm then a topper can be a very good way to add any additional softness and pressure relief to a mattress that you may need.
If you do decide to go in this direction then post #2 here and the topper guidelines it links to can help you use your sleeping experience as a reference point and guideline for choosing the type, thickness, and firmness for a topper that has the least possible risk and the best chance for success and also includes a link to a list of some of the better sources for toppers I’m aware of as well.
The choice between different types of foam materials is more of a budget and preference choice than a “better/worse” choice (assuming that you are only considering good quality/density versions of memory foam and not the lower quality/density versions). Some people prefer the more “in the mattress” feeling of slow response materials such as memory foam and others prefer the more “on the mattress” feel of other types of more resilient materials. There is more about how memory foam and latex compare in post #2 here.
Phoenix