The Nash is massive and very heavy, and that's without the 3rd skin (which IMO is a pretty pointless product). I'd avoid this unless you plan on doing long-stay static fishing (this is not the bivvy you want if you intend to stay on fish). Decent for long session holiday angling where you cannot move I guess. My mate had one and hated it, it's like building a house every time you put it up.
The M3 is right up there with the Armo and the Titan as one of the best all-round bivvies going. A very, very good shelter.