I'm not sure there is too much difference between one fan and another. The only big difference of course is the cfm rating, but blade shape does make SOME difference to the amount of amperage a given fan will draw at a given rpm/cfm.
Generally you want the biggest airflow you can afford, particularly if it is switched power. If you are tying your fan into a temperature activated switch, a manual switch, or an combo like kidtaurus is doing it is not as critical as it is if you don't. But in my book you cannnot have too much airflow when you are at a stoplight in the Texas summers (as long as your alternator puts out enough juice at idle that is).