I’ve built three mirrors so far. The first was with acrylic and the next two were glass. The glass is definitely the way to go. The acrylic warps a bit so you get a slight funhouse mirror effect, especially along the edges. I built that one in my garage, but it now lives in Florida. The change in climate made the frame expand and put pressure on the edges of the acrylic, making it warp slightly.
The glass doesn’t have this problem. (Although both of those mirrors are local).