I understand the desire to buy a small flash as the 6d doesn't have a built in one, you'd be mistaken for thinking you were adding something useful the designers missed out.
That's not really the case though. A built in flash is of very limited use, you really should go for a flash with a tilt and swivel head, then you can use it to bounce light and be creative. Straight on camera flash is a horrible thing that only rarely will add to an image.