Sleeping on air is gaining in popularity. Absent metal coils and springs, air beds use air chambers to control firmness by letting air in or out. Cheaper models have manual adjustments while more expensive models offer a host of features, including motorized air intake and infinitely adjustable firmness.
Perhaps one of the best features of air is that most bed sizes include dual air chambers, making it easy for each sleeper to adjust the air fill to just the right firmness.
Budget-friendly air beds like AeroBed can be quite comfortable, yet may be prone to damage, since air chambers are not protected by any covering other than sheets.
Higher quality air beds such as Select Comfort have air chambers completely enclosed within layers of foam, thus keeping them safe from damage inside a shell that looks like a regular mattress. These also offer up to a 20-year warranty.
Bottom line: Budget versions that completely collapse may be just right for occasional guests. For daily use, however, go with an airbed that looks like a regular mattress and comes with dual chambers. Check warranties and construction features as well.
Foam mattresses come in two general varieties. Inexpensive versions are often made of lower quality foam that tends to break down over time and may need to be replaced often.
More expensive foam mattresses, also known as memory foam, now offer space-age technology and a longer-lasting foam that is advertised to mold to your shape, then return to normal. Foam may need to be adjusted on setup to soften it up a bit; check manufacturer's instructions for specifics.
Foam is also affected by temperature, so it becomes softer under the warmer parts of your body and stays firmer where it is cooler. Some foam mattresses are also treated to be resistant to mites and other household allergens.
Some sleepers find the dents that form in the mattress as they sleep can take some time to readjust after they change sleeping position. You'll have to consider if this would be a drawback for your sleeping habits.
Some manufacturers also offer in-home trials, though beware of the time and expense involved in returning a mattress you don't like.