If you are not concerned about OW vs *A mileage accrual, to me this is a no-brainer. It has to be the non-stop TG flight.
I have not been on that flight myself but I have done CX in Y LAX-HKG. The flight was full and the seating was cramped. Thai are well known to offer better Y seat pitch in most of their aircraft, and I understand that the 345s they use for LAX-BKK have above TG average pitch. My experiences of other longhaul TG in Y have been favourable.
Both options involve very long flights and whichever you pick you will be very glad to get off at the far end. But I would think that TG would be quite a bit more bearable overall.
The TG Premium Economy gets good reviews if you could stretch to that.