Laser is a coherent source and produces a single wavelength. Different Lasers (Alexadrite, ND:Yag etc) are used for different skin colours.
IPL is incoherent broad spectrum light.
I would say that the majority of people experience better results with Laser although some do see results with IPL. The main reason is that all the energy of a Laser is contained in the single wavelength and deposited at a certain depth. IPL subjects the skin to a wider range of light energies of varying absorption coefficients, so it doesn't have the same destructive capacity as Laser.
Most clinics serious about hair removal opt for Lasers. IPL is mostly used for its low cost, although there are one or two machines that I have heard okay things about. Another reason is that Laser is quite painful whereas IPL is almost pain free. Lastly, one doesn't need many Laser treatments (if carried out properly) but IPL treatments are required more often.
you can read this for more info too:
CACI IPL Laser hair removal???