Just going by images the Sapphire has the larger heatsink, nearly triple slot. So I imagine it should run a little cooler.
ASUS has some higher stock clocks. So maybe a higher quality binning, or they have a more expensive vapor chamber or something under there that lets them get away with a smaller cooler.
An RTX 2070 isn't that much more, to be honest. With Nvidia now offering decent support for FreeSync monitors, I don't see much call for adopting the older card.