There are two major types of crib mattresses sold today. One type is polyurethane foam and the other type is an innerspring mattress

A good quality foam mattress will be as good for baby and last as long as a good quality innerspring crib mattress.

For Foam Mattress
The better foam mattresses are high-density, about 1.5 pounds (680 gm) per cubic foot (0.02832 cubic meters). Unfortunately, most foam mattresses don't list density on the packing, so it's hard to know exactly what you're getting.
Generally, the heavier the foam mattress the better, the firmer the mattress the better, the faster it regains its shape the better the foam

Innerspring Mattresses
What is important in an innerspring crib mattress is the number of layers it has and, the quality of each of those layers. The most important layers are the innerspring unit itself and, the insulator pad layer.
Coil count. The coil count for a quality mattress should start at 300 for a full or standard mattress, 375 for a queen and 450 for a king-size mattress. A higher coil count means a higher-quality mattress

WHAT TO LOOK FOR WHEN BUYING ONE
Is it the right size? If you can fit two fingers comfortably between the mattress and the side of the crib then the mattress is too small, which means your baby could suffocate if he falls between the mattress and the side of the crib.
Is it firm? Squeeze the mattress at the edges and center to test its firmness. Firm is good. Some cases of SIDS have been linked to soft bedding.