For reasons I will explain, I recommend the 50 1.4, but that is without knowing what kinds of pictures you'd like to take and how easy it will be to move around during a show :-) I suggest renting/borrowing both lenses from a local store, a friend, or a place like lensrentals.com and trying them out on a few shows before buying. You might come to prefer the shots you get from one lens vs. the shots you can get with the other.
But here are my thoughts:
f/1.4 vs. f/1.8
Under the same lighting conditions, the 1.4 lens will let you use a faster shutter speed than the f/1.8, allowing you to freeze onstage motion better. The usefulness of this feature will depend on how much your subjects are moving -- a little movement (singer/songwriter) will not require this fast shutter speed, but a lot of movement (heavy metal band) will. But my opinion is that having the option is always better than not having it :-)
The 1.4 aperture also gives you more creative possibilities due to the shallower depth of field. Depending on the sizes at which you post or print, you may not care about this aspect. But it's something to think about, and again, having the option is nice.
The Shots you Can Get at Each Focal Length
You say you like the 28mm focal length better, but each focal length is good for different kinds of shots. I'm assuming here you're on a crop camera (my experiences are with a 1.6x crop Canon 60D).
50mm is good for framing the head & torso (and part of a guitar) of a single person onstage. To do this, you will have to be at the very edge of the stage. In small venues this might not be difficult, but sometimes getting that close & and staying there w/out annoying other patrons is a concern.
Here is a picture I took at 55mm on a Canon 60D (so, 88mm effective focal length):
I was in the photo pit right against the stage. The singer was close to the edge but a few feet to my left. 50mm would be slightly wider than this, maybe necessitating some cropping to focus on the subject more, but still very good.
At 28mm, framing a single person's head & shoulders will be difficult, probably impossible at most places, unless you are OK with cropping. Whether you are OK with it or not depends on the megapixels your camera has and the final image size you're looking for.
28mm is good for shots of multiple people, maybe two guitarists jamming together, a singer closing their eyes while a guitarist solos, etc. In portrait orientation, you can also get good shots of a person's full body from head to toe, for example, a guitarist getting into a groove, a singer dancing at the mic, etc.
Without knowing a whole lot about what kinds of pictures you want, I recommend the 50 1.4. You have the option of using faster shutter speeds and can get close-up shots of single people when required, then move further back in the crowd to get a 'whole-band' shot -- although you will have to move pretty far back.
You don't have the single-person option with the 28mm, and the 1.8 aperture will require a slower shutter speed, leaving you open to motion blur.
These are just my thoughts. Again, nothing will help you out like trying these lenses out for yourself.