All the rooms at the Thalasso are beautiful. The only difference is the view, the rooms are exactly the same. I'd book the cheapest and count on a upgrade. On our reward stay last winter we were upgraded from the Emerald (cheapest) to Diamond (most expensive). Emerald faces back towards the resort and is less appealing, so you're taking your chances. But the difference between Sapphire and Diamond seems minimal. So you should definitelty be safe booking a Sapphire.

We had an amazing stay and found the RA treatment to be excellent, even on a reward stay.

One other thing to think about is whether it's worth it to use points at the IC Tahiti. I found a super cheap rate there (around $150/night) so it made way more sense to save the points for one of the more expensive properties.