The difference in the fabrics the shells are made out of has been discussed in past threads in great detail, so I would recommend trying to dig some of those up. In a nutshell, the MF fabric is much more water resistant, and the Extremelite fabric seems to get damp a lot easier. I personally like the MF fabric better, however 2 of my 3 WM bags are made of the Extremelite fabric…go figure.

As for the Alpinlite, I do not have direct experience with it but my good hiking bud has one. He loves his. He’s a warm sleeper, and doesn’t seem to have issues with taking it down to it’s rating. As Ken said though, it all depends on how comfortable you are in different temperatures.