I flew UA 767 J and LX A330 J back to back, both out of ZRH, last month.

In terms of seat and hard product, UA is much better than LX. LX seats are narrow, no privacy at all, and IFE screen is tiny. Even in the true window seat, which I was in, it didn't feel comfortable or private. Also, the bedding on UA is nicer, with mattress pad and slippers.

Food is better on LX as is service. LX has a rather large pre-order meal selection. Service is also better on LX. They have a pretty good system where the FA serving drinks will put your order in their iPad. Then a second FA brings out the dish.

Other stuff: Amenity kit on LX is not as good as UA, just a flimpsy Victorinox nylon bag. And wifi on LX is extremely expensive, 34.99 CHF for 4 hours on LX vs. 14.99 USD for full flight on UA.

As noted above, the only reason to go with LX is if you wanted to upgrade to F. But that's a gamble depending on the route, availability, and price.