Firstly welcome to the forum.
It's difficult to answer more fully without knowing details of type of venue, the lighting conditions and how far you will be from the stage (but I guess as a "friend" you should be fairly close without any access issues).
I would tend go with the faster manual focus 50 mm, you could always try and stand back a bit to get more in, subject of course to available space.
These lenses are readily available and can also be purchased second hand for well within your budget. Manual focus works well in these conditions, as auto focus can sometimes be fooled but various lighting setups and mic stands etc.
Watch your focusing, it ain't easy in low light, split prism screens can help, but a lot comes down to sheer practice and doing it for real. Don't be afraid to crank up the ISO, certain genres of music actually benefit from added grittiness that grain can give an image, Rock to name but one.