Well, for BD Diamond Club, SQ even in F only gives 200%. TG will give 300% on the HKG-BKK leg and 200% on the BKK-SIN leg. But that is such a minor consideration. I am a Skyteam flyer, so would prefer one of the airlines in that alliance, but China Southern is not so desirable. It's a 757 from Guangzhou to SIN. Hmmmm.. 744 or 777 vs 757? Is that even a contest?
Regarding the 5 hour connection, my SO will do whatever I choose. That is not a problem. And I imagine we would both enjoy the massages in the spa. That would really be the only reason to take the connection - to experience the spa and F class lounge in BKK.