Both LH and LX have less legroom than AC in the back cabin. You also get a small bottle of water with your main meal instead of just pours (which are also served throughout the flight). All three carriers provide comp'd alcohol in economy unlike UA.

Also, be careful what fare you book since many LH and LX low fares don't earn mileage in MP, the most common being the K fare. There have also been recent revisions to AC's economy fares which now earn variable miles in MP so consider that factor when booking on any of these carriers.