the alien bee ringflash is awesome...but its bulky and more of a studio ringflash than an event one. i can't imagine sticking the ABR800 into a brides face and blasting her with that. not to mention that it would require a vagabond to power it and your mobility with that rig becomes extremely hindered.
though i have no experience with the Canon MR-14EX ringflash i have some colleagues who have told me they use it for event type stuff. from what i have heard though, in many situations it lacks the distinctive ringflash "halo" look that you get with the larger studio rings.
if you run with an assistant (who could tote it around and pack it away when not using it) and only plan on using the ringflash for a few "signature" shots then maybe you could pull off using an Alien Bee. if you plan on moving about and using the ringflash a bit more extensively then i would recommend the MR-14EX.