You need to find the mattress that is most comfortable and most support for your body. The way to do this is to try each of the mattresses by lying on them for at least 10mins. It sounds like your existing base is a solid firm base, in which case you should try the mattresses on a similar base, platform top, to try to replicate the feel of your own divan.
You also need to be sure that the mattress you choose is exactly the same size as your base, especially given the fact that your base comprises of two singles attached together. Putting a mattress on a base which is unsuitable or for which it was not intended will invalidate your guarantee.