I believe that the M50 is primarily designed for racing. It's essentially two huge engines with a cockpit strapped to them (and two laser repeaters....which makes no sense for racing but hey).
The Gladius was designed for interception. It's two huge engines with a cockpit strapped to them....and more weapons.
At least that's how I think about them.
I think the confusion around the M50 has mostly come from CIG and how they've approached it.
Back when they first concepted it, it's original role in lore was an interceptor (it is, after all the "M50 Interceptor"). The concepts even had special dual barrel guns. Back at this point racing wasn't even a thing in SC.
EDIT: Found the old pic:
I think currently it's actually starting to fit into that role better. It's the best handling ship, and its size makes it very hard to hit. The balance is its shields and armor are quite weak, so if you
do get hit, it's over pretty quickly.
I think it has to maintain some level of combat viability to be true to the original backers who bought an "interceptor" at a very high price compared to other single-seaters. I think it's actually balanced pretty well right now, but only because the Tarantulas haven't been nerfed yet. I think once they are, you'll see this ship return as a shiny hangar ornament and the cries of the M50 Interceptor original backers will be heard once again.
You're right though, the Gladius is a pure interceptor. Right now it's too slow and sloppy, but they said they were going to fix that in the next big LIVE patch.