If you are doing a walk around shoot you might be in trouble. Here's my reasoning. Correct me if I'm wrong.
When I think "semi-formal dance" I'm thinking dim rooms with relatively high ceiling. Worst condition would be a gymnasium with ultra high ceiling. If it is really dim you will need a flash no matter what (direct, or bounced) especially with a crop sensor, despite the f/1.8 of the nifty fifty. In the worst condition with ultra high ceilings, no speedlight has enough power to do a bounce shot. You will most likely resort to direct on-axis flash, creating harsh lighting similar to those taken with a point and shoot. You could try getting a bracket to hold the 270ex off axis connected with an ETTL cord. It'll give nice off axis catchlights, and give angled shadows around their faces (still 'harsh' but better than on-axis).
Lens wise, I would choose the full range of the 18-55mm. 50mm is too long in a room full of people. If you move back for a group shot, you will most likely have a couple or two go twirling in between you. I would also shoot at least around f/4 to have deeper depth of field. I've done major blunders with the f/1.8 in the past. The front row being in focus, while the second row just half a foot behind them out of focus. With direct flash, you'll have plenty power even with the 270EX to pull off f/4.
As for the 100-300mm, same argument as the 50mm except now you will have half the room in the way I once thought "hmm this would be great for candid shots. They'll never know I took it!", but I just brought more attention to myself with the bigger lens and could never get a clear line of sight. I'd certainly leave that one at home.
...but if you were doing Portrait Sessions outside of the dance floor....well...that's a different story haha