Hi jovan,
Either one would work well in a suitable firmness level so the choice would be a preference rather than a “better/worse” choice.
I don’t have any personal experience using a soft latex topper under a firmer futon and there are too many unknowns variables, and personal preferences involved for anyone to be able to predict with any certainty whether any combination of materials will be a good match for you in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences). The Talalay topper is in a soft ILD range (19 ILD) which may be more suitable for use over a firmer mattress (or a futon) than under it but I would be aware that if the topper from Sleepwarehouse is 100% natural Dunlop and has a density of 4.7 lbs/ft3 then it would be firmer than the 16 - 18 ILD that they mention in the description (see post #2 here).
When you are buying a topper and you can’t test the combination in person to make sure that it’s a suitable match for you in terms of PPP then I would make the options you have available to exchange the topper for a different firmness level or return it and the costs involved a more important part of the “value” of your purchase just in case your choice doesn’t turn out as well as you hoped for.
I tend to prefer the more springy “feel” of Talalay vs Dunlop but this is just my own personal preference and other people may have very different preferences from mine (just like they may prefer different foods or different types of clothes than I do). I would also lean towards a softer topper over a firmer futon rather than under it and I would choose a topper using a combination of my own experience on the futon by itself along with the information in post #2 here and the topper guidelines it links to (which includes a list of suppliers that have good return/exchange options).
Phoenix