You don't have to reply if you don't want, no big deal... I get what you're saying.
Yes, "M" GPU's are weaker. We can agree on that. But they're also totally different hardware, that you cannot buy for a desktop. If you're out buying a car, and see a bike nearby, you don't ask "How much is that car?"
You cannot buy a 960M (Well, I guess you could probably find one on ebay), but you can't put it into a desktop. It's physically impossible. So that's not even a market consideration. Car sellers don't sell bikes. Video card vendors don't sell laptop GPU's. So no, you don't need to 'watch out' for the "M" ones, since you can't buy any.
And if you did, you bought a laptop instead of a video card by accident. At that point there's not much help.