I suggest avoiding on-camera flash like the plague, unless there's a white ceiling or wall off which to bounce that flash. For pics of two people, a fast lens isn't as helpful as you'd think - yes, you can keep ISO down with f/1.4 or f/1.8, but DoF will be thin, and prom couples would prefer to both be in focus, I expect. Of the two options, I'd get the 430EX II, but get it off-camera with a light stand, umbrella, and 'dumb' wireless triggers.
But I'd recommend neither. A one-Speedlite portrait isn't going to make many people happy. My advice - rent. Lensrentals.com has a two-monolight kit (with stands, wireless triggers, one softbox and one umbrella, for $133 + $35 shipping for a 5-day rental.
I decided the mono lights will be better for me. Now, one more question for you sir. As you may know the 6D has no sync port.. so would you suggest a hot shoe adapter with an extra PC sync port so I could use a moonlight with no triggers? Or another better alternative?
See if your local shops can rent you a profoto D1 air kit and you wont need the sync port. One local shop has them here for $100/day.