A couple of quick questions:
Is the place well lit? Will you have room to move around and photograph people?
The 50mm is nice, specially if the room is dark, but the major trade off is that you'd only get one viewing angle, and you'll have a harder time shooting around people. The 12-35 is better to grab groups of people, but if the place is dark or low light, a flash is going to be required.
I usually use the following settings for evening party shots: ISO 800 to 1600, 1/60 to 1/160, bounce flash if the ceiling isn't way up there, and my 24-70mm f2.8 (Sigma). Sometimes I'll put on the 50mm prime (also Sigma), if I can walk around the room easily.
Very rarely will I bring both lens along and change them to get a shot.
The photo below, of a friend, was taken at a bar this past November, using the 50mm lens, ISO 800, 1/125, no flash, DNG. Some minor Photoshop'ing to adjust colors and crop the photo.