The bren mag feeds more reliably than the stanag, and you retain lrbho and magazine release. As far as differences between stanag and bren, it would be the fastest to prototype with some dimensional changes. You can mill an existing XCR lower to fit them. We could probably design a steel lower insert that allows you to swap between bren (no insert) and stanag.Just my 2 cents, the bren mags would be a lateral move and not offer any distinct advantage over the existing stanag x39 magazines, I'm doubling down on my ak mag vote, for the mass availability. Also I like the price point of ak mags much more. $30 mags was a lot of what I didnt like about the existing Robinson x39, I don't think another expensive and hard to find magazine is the right solution (atleast for me), unless one already has a bren2 and is already invested into their magazines.
I agree that availability is an issue, but it's the cheapest shortcut in development time and prototyping vs the other options.