The futon mattress, in its original form, is a simple cotton mat that folds for easy storage when it's not being used to sleep on. The futon is placed on a thick woven reed base, which is then placed on the floor. Aren't you glad we've moved away from that.
The futon has evolved from a handmade cotton stuffed mattress that is only a couple of inches thick to a more comfortable sleeping solution that serves multiple purposes. Today's futon mattress is just as likely to contain cotton, wool, foam, and polyester. It's also probably made by the same companies that make traditional innerspring mattresses. Sometimes the difference between a regular mattress and a futon mattress may be in name only.
One of the defining characteristics futons is their light weight. It becomes a challenge to manage the balance between lightweight and comfort. Typically, any mattress with a thickness less than 5 inches will not properly support an average person's weight. That is, the sleeper will feel the slats or other supporting structure under the mattress.
One clear trade-off with the futon mattress is that it will feel harder or firmer than a traditional innerspring mattress, even if you don't feel the slats. This is because of it's relative thinness as well as the need for it to keep its shape when it's being used as a sofa. You obviously want the corners to stay square and up right. This is also a function of the material used to fill the mattress.
Of course, by definition a futon mattress has to be able to be folded so that it can either function as a sitting surface or be stored when not being used to sleep on. The thin all cotton futon is going to be the most flexible in this regard. Futons of other materials like foam will not fold as well.
All these factors come into play when choosing your futon mattress. Will it be used for sleeping only? Do you need it to double as a sofa during the day, while only occasionally being used for guests to sleep on, or do you want to be able to store it in a closet when not being used?
The 100% cotton futon mattress is going to be one of the heaviest for its thickness, because of the denseness and weight of the cotton. It's relatively easy to fold, and will become more and more firm as it is slept on. It's not the best choice if you plan to use the futon as a sofa during the day.
There are many different sizes including a twin futon mattress, full size futon mattress, queen futon mattress, king futon mattress and the styles vary and include tri fold, innerspring, organic, memory foam and others. Cheap isn't always best when it comes to mattresses but reading some revies certainly helps find the best bargain.
The foam cotton futon is an excellent compromise in that it reduces the weight of the mattress considerably. The foam also gives it more firmness in the corners so that it works better as a sofa.
Cotton/polyester combinations are also popular. They are lightweight, flexible and inexpensive. An all polyester mattress can weigh half as much as one made of cotton.
At the high-end of the futon mattress market is the all wool futon. Some people say it's like sleeping on a cloud. Wool provides great support while being lightweight. Wool mattresses are hard to fold, and therefore are not great if you want to use the futon as a sofa during the day. Wool futons are also quite expensive.
Interestingly, the latest development is futon style mattress which has an innerspring. This provides a sleeping experience very close to a traditional mattress. Not so sure that the Japanese would call it a futon, though.