Welcome from Southeast Texas!
If those are the only two choices, I'd say Yoder 640S (their new S controllers resolved many of my prior misgivings regarding the YS640, though propensity to rust still remains a concern). Having had hands on with both the YS640 and the RT 680 when I was considering my purchase, I found both of them wanting in certain areas and elected to go with a different brand. I think there are some other really good manufacturers out there including MAK, Pitts, and Blaz'n that are work a look that are within the same price range as the Yoder you're considering.