The best thing you can do is to personally sit on the mattresses and to lie down on them. Some people like the springiness of coils. Others like foam. Others like the feel of recycled materials like our EcoRest. They all have a different feel.
Some warranties cover materials and workmanship only.
You want a warranty that covers compression. One thing you can ask about is the density of the foam. Generally the denser the foam, the longer your futon will last. Choosing a cover is like choosing any upholstery fabric. Thicker fabrics will usually hold up better, but not always.