Things to consider (LAX-Germany is my commute, more than 5 RTs in F on LH and LX this year):
- Avoid FRA. Bad airport to connect, no privacy as long as LH has 16 seats on the 747 (which you are planning to do anyhow)
- The lounge in MUC is nicer than in ZRH. Getting from the lounge to the plane is equally bad (potentially long walk in MUC, the transfer to E is a hassle, two times security and you need to leave the lounge early).
- MUC transfers to/from the lounge when you arrive/depart on an outside position always work.
- ZRH arrivals at the E gates, normal immiration, train, walk to lounge takes significantly more time than MUC.
- No duck in the lounge in ZRH
- ZRH leaves LAX later. Better for sleeping. You avoid the worst traffic to/from the airport.
- Seats are similar, i find LX a bit (but not much) more comfortable on LX.
- Better blanket and pillow on LX. Better IFE (but not great)
- Better food on LH (but that is my personal preference, some like the Bratwurst mit Roesti and the better cheese on LX)
- Crews can be a hit or miss, but most of my bad experiences have been wit FRA crews. MUC and ZRH crews are normally good.
- Better german reading material in the lounge and on the plane in MUC. Very limited in ZRH.

Overall i would pick ZRH, but it is almost a wash.