Bipolar drives cost more money and are more complex, but are generally faster. Unipolar drives are cheaper, simpler, and slower, and less efficient, because you need to waste energy on heating the current limiting resistors.
I use a unipolar board. The FET3 from stepperworld. I love it. I use a 27 volt power supply and get over 40" per minute using 100 oz steppers and 1/2-10 acme screws.
If I did it again, I would go bipolar.
Eric