which one should i buy? or should i buy H-bridge?also, can i give PWMs to dc motor using these to make it run at different RPMs?
If H-brige is made of FET outputs, then you can control speed with PWM without annoying noise.This shield offers both (h-bridge and mosfet)http://www.adafruit.com/product/1438
Don't get the '520, its not logic level and hopelessly out-of-date component. Its also 100Vpart which is a waste for 12V circuit (get 20V or 30V part).Do you need bidirectional control? If so you need an H-bridge, if not one transistor/MOSFETwill do (but add a free-wheel diode across the load).