Wow, that’s a really tough question since they’re made for such different purposes!
The QC25 is harder to find these days since Bose took it off the market…so you’re in a position of trying to find backstock or buy a used pair. I would lean towards trying to find a new pair if I could. Not that I don’t trust used headphones…but if you’re buying used, it’s better to buy a pair that hasn’t been discontinued in case you need replacement parts down the line.
It’s a real shame that Bose took the QC25 off the market. They’re slightly more comfy than the already-nice HRM6’s, and the active noise cancelling means they have much better isolation.
The HRM6’s have a sturdier studio feel to them, and a sound signature that’s more versatile and more revealing of fine details in your audio. Their treble performance is much much better.
It really depends on what your usage scenario is. If you’re planning to do a lot of flying, or listening in louder areas, then the QC25 is a better pick. Its active noise cancellation and pleasant, long session-friendly sound are its main strengths.
If you’re the sort of person that loves to do critical listening, or you consider yourself an audiophile, or you plan to do any kind of audio production work…then the HRM6 is the easy winner. It has a better-balanced sound signature and still isn’t a slouch in the comfort department.
The fact that the HRM6 is still actively marketed is a big point in its favor, since you’ll have an easier time getting warranty support if you need it.