Edit: This post is directed to GBGT...
Short answer: 85RD
Long answer with preface: I have limited experience with Ultrafina as I only obtained 4 ounces from a trade. It's almost double the cost of comparable finishing polishes so I wanted to be sure of its capabilities before going all in. With that said, both Ultrafina and 85RD are quality products but I have found 85RD to have touch better finish. I have used 85RD with both rotary and DA (Flex 3401) without any issue. With the limited amount of UF that I had, I only used it with a rotary as recommended by 3M - Don't let this piece steer you away as I am sure other members have used UF with a DA and obtained great results.