Fellow Musician (I play in two bands) here.
At smaller / mid sized club venues, and for 3+ member bands
The 35 will have a lot more utility than the 50.
They both have similar light gathering power (even though the 35 is slower, it has a much wider angle of view)
the 35 will be a little more forgiving with focus / DOF
For larger venues (or where you cannot be close to shoot) or smaller 1-2 member bands:
The 50 may be a little better.
Also the 50 can also be better if your subjects are really still and don't move much, you can go for shallow DOF.
Here's a
sample vid with 7 camera angles of which 3 are Pentax: K-1(31/1.8) K-1(DAF50/1.4) K-01(8/4 fisheye (de-fished))