The main difference is in the cooler. Founders edition cards run warmer (as do almost all reference cards, in fairness) - there were reports of thermal throttling after release, and I haven't heard anything further - so I assume they still suffer from that somewhat.
If your card is throttling, it'll reduce it's power (and speeds) to reduce temperatures.
Aftermarket cards (I assume you're talking MSI 1070 GAMING) have improved cooling solutions, allow OCing headroom if that interests you, and should not experience throttling with decent case airflow.
I don't see any justifiable reason for a Founders 1070.........unless it was half the price of the aftermarket cards (which it's not) and you could put up with throttling.