If your CPU can handle the faster one, go for the faster one.
Meaning: Try and figure out where the bottleneck will be when getting the card with more memory, and if it is the card (and not the CPU/Memory) then go for the faster one (Dunno how to do that excactly, but there was a site that might be able to show you?)
Besides that scientific approach: if you're only on 1080p, go for the cheapest option. My 1080ti 12GB runs GBS and DCS fine on 1080p. DCS maybe not on all full details, but I do not care, don't have time or braincells to spare to look around how nice everything looks whilst trying to keep them birds in the air, and navigating and weapons systems and stuff... )