I have the 554 and its pretty awesome
I havent looked through S.3A so cant comment but the 554 is better optically than the S.2
Last year I ran the 554 against some similar size scopes like the S.2, the Nikon, the Vortex and the Leupold and the 554 is better in my opinion. Ran it side by side with the Swaro STC last week and thought the swaro was maybe a little better over all but not optically