2 cents from someone who has flow LH Y 20 + times in the last year or so...

Go with United if you can. If you can't, ask for an exit row at check in (get there early). Also, you can check in by phone and lh.com is much better about online check is as well.

If you are medium to large, I recommend middle aisles - check the seatmaps online. LH doesn't block them, but I have very good luck getting an open seat next to me if I book 33G or something similar on a 340. The window aisle on most LH planes has no foot room and always feels cramped to me.

On the 3 5 3 configuration, middle aisle is still the ticket IMO. LH Y has 0 leg room and your best bet is an open seat to your left or right. I would also recommend that the first thing you do is put the inflight magazine and other miscellaneous items in the overhead bin, since this will give you knees much more room.

All that said, good airline - great food, free booze, etc. Inflight entertainment is a bit lacking, but I'm guessing you will want to sleep for the first 3-4 hours of the flight.

One other trick - get a special meal and they won't mind if you sleep through the meal service and want it later. This is harder with standard meals in my experience.

Enjoy!