For walkabout/hallway shots in a crowded convention, 50mm is gonna be a little on the long end for full-length shots, but will be perfect for portraits, head-and-shoulders, etc. Of course, when you're that close to the subject, perspective distortion is going to prevent you from getting pleasing full-length shots anyway.
For photo sessions in a controlled environment, the 70-200 is my go-to choice. Stand far away to minimize perspective distortion, unless I intentionally want to include some perspective distortion.
Shooting the stage from the audience (e.g. costume contest), the 70-200 is definitely a good choice.