Hexcrews has a link regarding strength but they don't mention 12.9 -look here
I know from experience - they last longer. I had ss which look good but wear out much faster (like butter ;) ). Thats why all of our kits come with these alloy screws.
Thats whats killing hexcrews, they don't mention that their screws are 12.9! The only way i know they are 12.9 is because they ingraved it on their screws.
I can't stand stainless steel, i always bend it or chop it off. If not that, i'd strip them.
I usually buy from www.tonysscrews.com . They have great screws, but they make their flat heads in 10.9