I'm UA 1K, LH HON, and SQ PPS, so I've thought about this question a fair bit.

SQ is much, much more generous with awards on SQ metal for PPS customers, which you surely will be (since you can get that in addition to UA or LH miles).

So even though you need a boatload of SQ miles to actually get an award, if you want SQ award seats for someone else to accompany you, you might suck it up and credit to SQ. The other gotcha is SQ expires miles so there is no guarantee you'll ever get enough for a free ticket. The expiration bothers me a lot because I do not know what regions I will travel from one year to the next so I stick with UA and LH for crediting rather than SQ.

SQ miles in UA will count toward 1K. SQ miles in LH will count toward SEN. SQ miles do not count toward LH HON, in case you might covet that.

The big advantage of UA M+ is that you earn many types of upgrades. LH SEN get almost no upgrades for free (1 long haul segment every two years).

C and F earn more in LH M&M than UA M+ but awards also cost more (but are more available). C and F travel on SQ will likely get you more awards using LH M&M rather than UA M+. Plus they will be easier to book.

If the UA upgrades are worth enough to you, UA is probably the program to use. If the faster and easier awards are worth more, then LH is probably the program to use. If SQ metal awards are really important and you will fly enough to redeem them, you may want to use SQ's program instead.

Your SQ PPS card is *G so you get UA lounge access and all that just from getting the SQ PPS. So you don't need that benefit of LH SEN.

Charles