Don't know about the only 1/2 mag difference, but in real world observing experience, the 50mm can bring in many DSOs whereas the 30mm brings in few. It's the difference between knowing the object is there vs. guessing that you've found the right asterism around the DSO.

The light-gathering area is almost 3x as much in the 50mm --wouldn't that make the visual magnitude difference be over a full mag? Or is that not how that works?

Weight is another matter; the 30mm are much lighter. I used to be a staunch advocate of 6x30mm finders, especially because of their wider fov... until I started using a 9 x50mm finder! Under dark skies 6 x 30 might work fine, but in semi light polluted suburban skies, it's a different matter as the 50mm show you much more.

Dan summarized this well-- with a red dot or red circle pointer, go 9 x 50. In fact, given how lightweight red dot pointers are, and how versatile they are in use, they're highly recommended.