Hi!
These are two very different bags. The Alma BB is much smaller and holds significantly less than the Pochette Métis, even though it holds way more than one would think by just looking at it. The PM can also be worn over the shoulder, but the Alma BB strap, which is not adjustable, seems to be too long for most people to wear it this way, being it mostly crossbody or handheld. Their price points are also significantly different, especially if you're referring to the Alma BB in the regular canvas.
They are both absolutely awesome and gorgeous bags, even though the PM has some known issues, like cracked glazing.
I believe the Pochette Métis does have the potential to become one of LV's iconic bags in the future, but before that they HAVE TO address and solve all the known issues this bag has.