Firstly, you can't go wrong with either one. I bought a Superformance in 2001. I bought new and I didn't want to wait. This was a big plus in the Superformance column. If you are buying used, once you find the right car, you won't have to wait for it to be built, so that negates the immediate gratification advantage of Superformance.
If you are hung up on accuracy of your replica, the body shape of the ERA is a bit closer to the real thing than the Superformance. There are lots of things under the skin on both of them that aren't even close to an original. But we all know that if you are an accuracy freak you won't be happy with anything other than a continuation or a Kirkham.
I have heard nothing but good things about the quality of ERA and the folks that build and support them. But that is all second hand info for me. What I do know first hand is my experience with my Superformance. I am going on 11 years of ownership. I am on my second engine and have 62,000 + miles on the odometer. The car has been absolutely bullet proof. It is as tight and rattle free today as the day I drove it out of the Olthoff's shop. (An aside, I don't know where you live, but if you are within one day's drive of Mt. Ulla, NC, that would be a huge reason to put Superformance near/at the top of your list. Mt. Ulla is the home of Dennis Olthoff's shop. There is NO ONE that knows Superformance roadsters better than Dennis. The level of support and mainatanance available at his shop is reason enough alone, in my opinion, to own a Superformance.) I have drag raced, road coursed and long range road tripped my car. I have a wall full of trophy plaques I won in several years of Run n' Gun competition. I cannot imagine any replica could be more fun, reliable, trouble free and totally bad assed than SPF1061. That is my honest, personal and considerable experience with one brand of Cobra replica. My guess is that whichever you get, you are going to have a ball! Enjoy the ride!