The SQ bed is 73 inches long on the A350, and a chunk of that is in a tiny narrow offset footwell unless you're in a bulkhead. I have no desire to fly that again, and I think for anyone over 6' or thereabouts, it's a mediocre hard product.
Ergo, I'd choose based on your height. If you're under 5'10" it's a no-brainer to fly SQ. If you're over 6'2" it's a no-brainer to ... fly JAL in the apex suite
if you're in between, then it's your call I think as to whether the hard product or the soft product matters more.
You will, as has been noted, earn more miles on UA, even with the cap. PQP will depend on the fare.
Edited to add - I don't mind the firmness of the SQ bed, unlike a lot of others, but they're right, if you do, then just stay far away from the SQ A350.