There are two types of foam pads: the egg-crate variety, or ones made from memory foam. The egg-crate style is less modern than the memory foam type, but it's also cheaper. Memory foam mattress pads, however, may do a better job of distributing and supporting body weight. Denser memory foam will also offer more overall resistance to weight than an egg-crate, which flattens with wear.
A memory foam pad will usually cost more than an egg-crate, depending on the quality. If budget is a problem, it's more probable that you'll find an excellent egg-crate mattress pad for the same price as a mediocre memory foam model. Shop around to find the best comfort value for your dollar.
Egg-crate mattress pads are very popular in medical environments like hospitals, and for patient sickbeds, because the rises allow for good air circulation around the patient. This, in turn, lessens the chance of bed sores. A memory foam pad, though, is in contact with the whole skin area, warming around the body. If this pad is for someone who is bedridden, an egg-crate is probably best.
Because a memory foam mattress pad is in contact with all of the body, temperature comes into play. If you sleep warmly, a memory foam pad may feel stifling, especially if you share the bed with someone else. An egg-crate, meanwhile, is better for a cool night's rest. People who toss and turn may find that the memory foam absorbs their movements better than an egg-crate, making your nightly exercises less of a bother to their bed mate.