We were at GH Erawan in May for 3 nights on Corporate rate (reservation for 2), TSU was applied last Sept when I was here. There were 3 of us, my wife and college grad daughter.
At check in they scanned all 3 passports and granted all 3 of us GC access and breakfast in the restaurant. Roll away bed was requested for the corner Grand Suite King. Nothing was said about 3rd person fee and no extra fees at check out.
We later transited Bangkok and stayed at SGS for 1 night, using my SPG (now Bonvoy) AMEX anniversary free night cert. I was charged 1900 THB for 3rd person, which is ~$70 all in. We were upgraded to a Luxury Suite (I had requested during pre-arrival email exchange) with roll away bed. Happy Hour at Barsu was great as usual, although bar snacks are just that, not nearly as good as GH GC, but much better cocktails. Breakfast at SGS was superior and more extensive. I was prepared to pay the 3rd person fee, and I appreciated the Titanium benefits applied to 3rd person as well.
Summary: Staying at GH Erawan saved me ~$210 for the 3 nights vs SGS. I would rate SGS slightly higher than GH Erawan.