Well, it's a very personal preference. Personally, I would choose 66UM over 80. Check the specs out if it helps.
There is no such thing as the better string as this is personal preference. Those charts are for marketing purposes and do not represent real world performance.
It depends on that racket you're using and probably your play style preference. So if you've multiple same model of racket, string one with 66UM and another with 80. And you'll come to a conclusion of which you would prefer after a handful of sessions.