I shot STI Edge for a while in USPSA and upgraded to a steel grip custom gun. It really tamed major power factor and let me be a little less focused on my grip and still get good hits at speed. Custom gun smoothness and fit will last much longer if you're planning to shoot high round count 100K over the life of the gun. Worth twice the cost? I think it's more the pleasure of shooting/owning a custom gun was enough to push me over the edge. It made me a better competitor too.
STI is "factory" built where Atlas is one off. You will find similar differences in STI vs SVI (Infiity). STI guns are nice but in custom builds like Atlas or SVI you will more attention to detail on fit and finish. Both will work just fine in competition.
Atlas is noticeably nicer, but its worth trying out a couple and deciding for yourself. if the STI fits and is tuned to do what you are doing with it then you just saved 2-3K it really is a smallish difference for 2x the money.
UNLESS that means something to you. 10% faster lockup or 15% less felt recoil ect....