It is unclear what your question really is.
Are you trying to book a ticket with miles? If so, do you have miles on Skyteam?
I f you have tons of UA miles and no ST miles, then it might affect your choice of airlines.

Now regarding quality. Y is Y and differences are only marginal in terms of seat comfort, food/wine and IFE.
On the route via FRA, UA and LH collaborate closely.
I believe that your statement that there is no IFE in Y is totally wrong. UA currently use a 2-class 777 with flat beds in J and 9 across in Y, with AVOD. They should move to a new 767 in the summer, also with Avod in Y. LH A388 has AVOD.

To summarize: in terms of comfort UA/LH wins as AF is using 10 across on its 777. In terms of food/wine AF wins over UA maybe not LH. In terms of IFE it is equivalent (although the UA avod system might be better).
Also note that the transfer at CDG between terminals might not be great.