Thanks for the feedback folks. Unfortunately it's too close to the flying date to get a SWU converted to paper for use on LH (and the LH C cabins look pretty full anyway).
It sounds like there is not much between them, which is good news in that I can't pick the wrong one as such.
(In spite of several years using FT as lurker and poster, I still occasionally book a hotel or flight without checking here first, then check, then regret ...)
I get connections either way, but staying on UA TATL means some awkward timings.
In the spirit of all things merger, I might try CO for the experience.