To answer your question about which is better
as far as quality?slate w/out a doubt,It has a life of around 50 to 100yrs.I've done work in an old neighbourhood in Princeton,NJ and some of the houses there built in the late 1800's still had original slate roofs!Slate roofing is definitely not a DIY,difficult to cut and apply.Synthetic slate(some manufacturers warranty for 40yrs) is also expensive and difficult to apply a job best contracted out(But man it's gonna cost a fortune!)