This will depend on what you need to be cozy at night. For those who benefit from or prefer a harder sleep surface, a futon mattress could be just what you need to achieve a restful night’s sleep. A standard mattress offers more variety when it comes to firmness, construction, and materials so it’ll suit a broader range of sleepers.
If you're tight on space and/or cash, a futon could make the most sense for you. Fortunately, you can make it softer by adding a separate mattress topper, but you'll have to remember to remove the topper each time you convert your futon back into a sofa.