If you fly UA a lot, you might want to join LH's M&M.
LH's miles can be used to upgrade UA flights (with restrictions).
LH status last for 2 years is still a big plus. They are also more compassionate for various leaves.
Also, you can still enjoy 25% bonus on UA flights after you reach FTL.
If you fly SQ a lot, then you might want to join LH's M&M and SQ's PPS. Put the SQ sectors in PPS and miles to LH's M&M.
Which parts US do you fly to the most? (east or west coasts)