i switched to the 130mm alias
it is a bit less comfortable than the wider seat but i can get off the back of my bike easier when going down hill
you are on/off the saddle way more on a mtb than a road bike i would think. so it is not really an issue.
i havent heard of an actual butt measuring device but if there is one, it wouldnt surprise me.
i would say borrow some friends saddles that are different shapes and sizes and just go do a long ride with them to see might be the only way to really know.
when you are paying for a $100 carbon titanium saddle, you want to make sure it fits first.