The correct answer is “none of the above”. Alaska credits SQ business better than UA or LH. Not a *A program though.

https://www.wheretocredit.com/singapore-airlines/d

LH miles can expire as mentioned, and AS has interesting partners to redeem on (JL, EK, CX) so IF you are in a position where you could keep an AS account alive with regular earnings, you might consider it. It would be a bad choice if those miles get orphaned though.

Otherwise I would probably just credit to UA , collect your bonus, and be done with it.