Booked here in November under FHR 3rd night free. I asked the hotel and they confirmed the FHR contractual one-category upgrade from premium view twin is to the executive corner twin which is a substantial upgrade. It's a bit of an odd gap in their room categories as there is (no longer) a normal twin executive room category so this seems like the clear best room type to book if you want the best possible upgrade even if you don't need two beds.

Will probably double book with St. Regis and see if either decides to pre-upgrade in the days prior and do my own guessing on chances of an upgrade based on rooms for sale to decide where I'll go.