I am so glad I asked. The 77W actually has a better schedule for me than the 787-9. I had incorrectly assumed that the newer aircraft would have better seats. Thanks for setting me straight.
UA's 77Ws are newer, in aggregate, than their 789s. UA waited a long time to order the 77W.
Having said that, if you can get a bulkhead center column seat on the 789, I find that more comfortable than the consensus best 77W seat (odd bulkhead window). It's less private, but I actually prefer that, as the 77W J seat is
tight.
How are you getting back from FRA? If I had to choose one direction in which to spend the 110K miles, it'd be ex-FRA, because the First Class Terminal is a really cool experience.