As is so often the case this depends on the type of futon that you are intending to purchase, in particular what combination of base and futon mattress you are looking at.
Firstly....the futon mattress. If you are intending to use your futon as a bed and a sofa then you need to get a combination of comfort for sitting / sleeping, flexibility for folding into a seat and rigidity to ensure the futon mattress holds itself up when in a seat position. With this in mind I would always look for a futon mattress with a good quality 'core', such as foam or rebound polyester. The core should then be surrounded by layers of woolfelt and cotton. Generally an 8-layer futon mattress, weighing upwards of 20kg would make a good purchase whilst providing a firm, but supportive, sleep. These types of futon mattresses are available from all the independant shops who are Futon Blog members, as well as other outlets on the web.
If you don't need to fold your futon mattress, as it will remain flat on a bed frame, then you could also look at traditional 8-layer cotton / felt futons and the new 'FutoLatex' which contains a mix of cotton and 'cotton-rich' felt around a core of Natural Latex. These type of futon mattresses provide a slightly softer and supportive sleep, making perfect futon mattresses for an everyday bed. Generally these type of futons are only available from specialist stores.
Secondly...consider the type of base you require. In my experience the best bases for a permenant bed, if you are intending to leave your futon in a 'bed' position, are good quality pine bases, such as the Auckland or Osumi. They are simple, robust and will stand the test of time very well, we frequently replace futon mattresses for people who have had their bases for upwards of 10 years! If you want to move your futon from a bed to a sofa then go for a 3-seater style base, they are easy to convert. One or two retailers even stock 3 seater bases that use a 'sprung' beech-slat system for the seating / sleeping platform, providing extra comfort for regular use.
Anyway I could go on forever........so post any questions you may have. Bye for now.....
Follow this link to Futon World for a great selection of good quality Futon Beds and everyday use Futon Mattresses:
Futon Beds for Everyday Use