I just feel a lot faster with the single-stage. Kyle Lamb recommended a single stage to the point that he got Bill Geissele to make one just for him. So I use it. I like it. It works.
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I can see maybe why the SSA and -E are $45ish+ more expensive. The pin for the hammer uses a little coil clip thingymajig that keeps the pin from walking out. The SSA and -E do not need this and the pin is help in place by the hammer spring I believe.
Some differences in how they inspect the triggers. I think everything but the G2S are all checked individually (every single one of them) while the G2S are spot checked.
I think that is the differences but not 100% sure.
I would spend the $45ish+ the next time for sure and not thing twice about it. Just skip eating out for a couple of days at work is all.