While you're looking at the material of the soles of your prospective new cycling shoes, it's worth looking at the type of cleats that they will accommodate, as some will take two-bolt cleats while others will take three-bolt cleats. Some, particularly lower priced models, let you fit either. Your choice really depends on what type of riding you're doing.
If you're riding off-road or want to have a pair of shoes that are easy to walk in, then go for a pedal system that uses two-bolt cleats such as Shimano SPDs or Crankbrothers Eggbeaters.
However, for road riding, you really want three-bolt cleats which will give you a wider platform for improved power transfer through the pedals. For that reason, mid and high-end shoes will only come with three bolts patterns.