1. Where are you installing the fan? If it's replacing a light fixture, there are at least 2 things to consider: Can the box support a fan, and will you need a fan with a light kit
2. Look at the CFM rating, which measures air flow created by the fan. Saves you the trouble of comparing different number of blades, lengths, angles/attitudes, and speeds. Higher CFM rating means more air movement.
3. Longer blades are not always better, especially if the room is small or if there are sprinklers on the ceiling close by. You can still get a high CFM with a 42" blade though it will of course be easier to find high CFM ratings on 52"+ blades
4. Assuming you have a flat (not sloped) ceiling there's an option to mount the fan flush or to use the downrod to lower it if the ceiling is high enough for that (may slightly improve airflow)