How do I choose which kind of shorts to wear for mountain biking? There are so many different kinds of shorts available for mountain biking. 

I would like to get a pair that is:
durable, can have padding shorts (either insert or worn under), stretchy and flexible material, doesn't make a huge amount of noise when I pedal, lightweight, quick drying.

Usually I would wear just lycra but having done that last season and taken a couple of falls with no protection I'm looking for something a little more protective.

There are so many kinds of shorts out there though how do I know what is purely fashion and what is function? and is it worth wearing anything but lycra for xc?
I wear baggies for non competitive XC. Just like baggies on a road bike is a fashion crime, Lycra and MTB is a fashion crime (unless you riding at national competition level events) :) 

I have two sets of baggies - one is stretchy, light, quick drying and surprisingly durable -  think light weight running shorts with Lycra added. As they stretch, they are closer fit than many baggies. 

The other is less stretchy and heavier fabric, and baggier(Fox brand), but still not heavy and dries reasonably quickly.

Go to you LBS / Sports shop and shop around. You cannot do this on the internet as you need to feel and see the fabrics.