50mm prime or 17-70 for party photos??

[DzRbenson]

Since you have a flash, I would go with the 17-70, The 50mm would get better shots but restricted, however the 17-70 allowed you to play with angles.

[TOM]

50mm is my pick, it will have two advantages. Firstly the image quality is better and secondly, it is going to be two to four and a half stops faster! If you want wider, then take a few steps back. If you want closer, then approach the subject. Getting good candid shots is about making your subjects feel comfortable. Shooting from across the room with them not knowing your are there is not a good plan of attack. And pointing an SLR with a huge zoom lens on the front can seem intimidating for some people. The 1.4/50mm will allow you to isolate your subject by using the big apeture of the 1.4 and also some nice low light shots.