There are several important factors to consider when purchasing any piece of laboratory equipment, and centrifuges are no exception.  Capacity, speed, programabillity, G forces and compatibility are common factors to take into consideration. 

1. Capacity: How much?  if you have larger samples, such as a plethora of 50 mL tubes that you would like to run at the same time, full sized models would probably be best.  Alternately, you would use a smaller benchtop microcentrifuge type for spinning capillary tubes for a mobile clinic.

2. Programability:  While many centrifuges are programable, many are not.  If your application requires variable speeds/forces, then a programable centrifuge would be a great fit.  If you are performing repetitive centrifugations, in a common field, you may be able to find a more economical preset model. 

3.Speed/G Forces:  Know the correct speed/g force for your application!  Some of the more economical centrifuges will not be able to reach the speeds you need.  Be sure to also check the speed/force ratings on your centrifuge tubes.  A few of the faster centrifuges require special vessels to hold up to the speed/force!  Fortunately, the majority of them fall somewhere in between as long as they aren't specialized for a certian application.

4 Don't assume it comes with a rotor: Most centrifuges are sold without a rotor of any kind.  If in doubt, don't hesitate to ask the seller.  This is for your convience, as they do not want to charge you for a rotor you will not use.  The majority of centrifuges have several rotors available to choose from, ranging from very specialized to as general as a test tube rotor.  Be sure to check the speed/force ratings to be sure your rotor will hold up.

5. Temperature Control:  Some centrifuges are even available with refrigeration (usually programable). 

Finally, check the voltage/plug to make sure the centrifuge is compatible with your electricity supply.   US appliances use 110 to 120 volt, 60 Hz power.