The OP is flying ex-Europe, if you check the post.

I would see some very good reasons to fly LH F instead of UA F ex-USA, though- if you have to connect in SFO/LAX/ORD/IAD to get a UA nonstop to Europe from a hub (like me, SEA-based) and then (possibly) connect AGAIN in FRA/MUC, as opposed to taking a nonstop/single connection, it would be a no-brainer for me- I'd take LH F to Europe without hesitation. The difference between a 15" screen and a somewhat better lie-flat, and a 7" screen and a worse lie-flat is not enough for me to not like the convenience of the nonstop on a longhaul, especially given the other advantages that LH has.

That being said... I'll take a connection if it times the flight better. I preferred HKG-ZRH-FRA over HKG-FRA on a trip because the LX flight was a night flight- it's easier for me to get a decent night's sleep in LX F then than during daytime on LH.

The bottom line is there is no right answer that will suit everyone. Everyone's needs are different, so pick what suits your needs.