Hi jomiller,
I’m assuming that your daybed doesn’t have a pop up trundle attached which you want to “match” to the mattress on the daybed (to make a short king). I’m mentioning this because it would limit the height you would be looking for to what would fit in the trundlebed (usually under 8") although 8" or less is a good height anyway because more than that and it may not be as comfortable for sitting.
I would probably be looking towards a latex/polyfoam hybrid with 2-3" of Dunlop on top (doesn’t compress as much as talalay with the same weight) and a firmer high quality polyfoam underneath both for good support. The polyfoam is stiffer and not as point elastic as latex and will also reduce the cost for the more occasional use of a daybed. With a hybrid construction … and if you also needed a softer surface than Dunlop would normally provide … then Talalay would also be fine but would compress a little more than the equivalent Dunlop layer but be more pressure relieving.
The specifics in terms of layer thickness and firmness levels would be the same as a regular mattress with the thickness and firmness of the various layers depending on your body type and sleeping style. Depending on how much sitting you were doing vs how much sleeping … I may also modify your choices towards slightly firmer if you will be sitting more on the mattress.
Another alternative depending on your budget would be a solid 6-7" Dunlop mattress that was soft enough for your sleeping needs and preferences. In this case … 2 sided would be a bonus because you could both flip and turn the mattress if you tended to sit in the same spot all the time which would lengthen the life of the foam. Ikea has an option called the Sultan Edsele which is widely available and better than average value for a mostly natural Dunlop latex mattress and it can be tested anywhere there is an Ikea within driving distance although it may be on the firm side. Local manufacturers may also have similar styles that can be tested locally and may offer a wider range of firmness levels and there are also several of the online members in post #21 here which offer a 6" and 8" latex mattress of various types.
Dreamsoft Bedding Ultimate Dreams mattresses would make a good hybrid choice because they can be customized for firmness and are very good value although they are 10" high and have a quilting layer on top which may not be as good for sitting.
http://www.latexmattresscompany.com/twin-7-zoned-natural-latex-mattress.html would also be a very good choice if you wanted to sit or sleep directly on the latex and it’s also thinner at 8".
Hopefully this will give you some good quality and good value choices in various budget ranges.
Phoenix