I would happily go to a Six Senses. But probably not the one in the Seychelles. However, my reasons are not those of the posters above. Rather, they have to do with the resort itself. SS is on a private island, but my understanding is that the island gets a lot of boat traffic from non-resort residents, thus impacting privacy. Plus, Seychelles is all about the beaches, and the beaches at this resort seem small and underwhelming to me. The villas looks nice though.
FS Desroches is not a new resort. Rather it is an existing resort that FS renovated. Considered a trip there this previous May, but could not get over how old the 'new' villas looked. Plus, I have heard conflicting reports on the quality of the beaches on the island.
If I were to pick between the two, I'd probably go to the FS. Or I'd just go back to the FS on Mahe, which has lovely villas and a wonderful beach.