In February I visited the following hotels in Thailand:

FS BKK
Capella BKK
The Siam
Amanpuri
Trisara
COMO Point Yamu

The Siam is a fantastic property but quite far from the centre so you'll be reliant on water transportation or beholden to the traffic.
FS BKK is phenomenal, with the exception of the breakfast, which was rather pedestrian and uninspiring. JMG has done a remarkable job with the architecture.
Capella next door had better service and the living room concept was wonderful. The public spaces are not a patch on FS though - the pool in particular is markedly inferior.

COMO Point Yamu is not a luxury property for the purposes of this forum.

Trisara vs Amanpuri

There is no question in my mind that those on a constrained budget should favour an entry level Ocean View Junior Pool Suite at Trisara over a standard Pavilion at Amanpuri.
Trisara rooms have complete privacy and all have pools with direct sunset views.
The standard Virtuoso upgrade (which I'd generally expect to clear for our clients outside of festive season) is to a 240m2 Ocean Pool Villa.

We stayed in a Partial Ocean Pool Pavilion at Amanpuri and were surprised at how many people's gardens and pools we had direct lines of sight into.

Trisara feels like a much smaller boutique property whereas Amanpuri feels massive (because it is).

Service is undeniably better at Puri but we found Trisara staff to be engaging, friendly and proactive nonetheless.
The beach is better at Puri but it is also very busy and separated from the main hotel buildings via a giant staircase.

I really enjoyed eating meals right on the water's edge at Trisara.
Having a pool right by the beach, a couple of steps from the shore was very convenient.

Moving from place to place at Puri felt like an event that needed to be planned, whereas at Trisara everything was moments away and didn't require planning.

The exception to this was the Michelin starred restaurant (the only one in Southern Thailand) which we missed out on, as we didn't reserve in advance.