Appetite for risk, threat model, competency available _and_ platform constraints, IMO. I think we're in violent agreement: you should only do this if you have someone on board who knows what the hell they're doing, and you literally can't afford a decent TLS stack on that device.
Comparing between those two options is a question of appetite for risk, your threat model, and the level of competency you have available. On the latter note, _at the moment_, I think the subset of people who are capable of taking advantage of Noise would be largely similar to the same subset you could safely task with designing and implementing a custom AKE+messaging protocol from scratch anyway. I think the level of competency you need available is on the very far end of the spectrum. Edit: and on the risk point, even if the library has been eyeballed less, at least you know with mbedTLS that if a bug drops you'll get a patch pretty quickly. If you goof up your Noise impl you're on your tod.