If you have to pick between J on a UA 787 or LH A380, I'd pick the 787 for the (1) smaller J cabin and (2) reduce risk of getting one of the D/G seats on the A380 which create "footsie" situations - but I think it's probably a really close call as I prefer the service on LH. Are you deciding between SFO-FRA-xxx (LH) and SFO-MUC-xxx (UA)? Ultimately I'd pick the shorter itinerary.