Hi ineedrem,
For many people this is an ideal combination and some of the most knowledgeable people I know who could sleep on any mattress they chose to sleep on an innerspring/latex hybrid. Innersprings have a different response than foam and can also have multiple spring rates and for some people it just “works better” for many reasons that can get quite technical (see post #2 here). While the choice between an innerspring and an HR polyfoam core would be personal preference (support layers don’t tend to be the weak link of most mattresses) and the polyfoam you are mentioning is very high quality HR polyfoam … you would certainly be in some very good company if you went in the direction of an innerspring/latex hybrid.
I don’t know the type or details of the innerspring you are considering. Can you provide more information or a link to the details? You also mentioned that the innerspring is two sided … does this mean that the latex would be a topper over a mattress that you can flip or that there is 3" of latex on each side of the innerspring (which would both be a bonus in terms of durability)?
The support layer is not likely to be the weak link in terms of the durability of a mattress and the polyfoam is very high quality so I would treat them as equals and make the choice based on feel and performance rather than durability all other things being equal.
Phoenix