This is a great cover. For one, the 'looking up' point of view on comic covers are usually done to make the hero seem bigger than life, to increase the feel of power. Here it shows the exact opposite image, a helpless Supergirl pinned to the wall. But I also love the use of the blue sun as the source of light in the background, slightly washing out Kara's face, the shadows deepening at Supergirl's feet. And the overwhelming blue tint from the sun just pops. Adds to the melancholic feel to the cover.
Just given the material, I also think this will stand out a bit on the rack.
This was the original cover solicited for Supergirl #6.
And while this is a very nice cover of Supergirl fighting with Reign, it is the sort of standard battle scene cover, probably lost amongst the other battle covers on the rack.
I think the new cover is overall a better piece of comic art. Hopefully, if a trade gets made of this first arc, this proposed cover will be included in back material.