FS Downgraded Our Room--not happy. haaaalp

Just got back from 4 nights at a FS Vail. We arrived at 6:30 PM, it was very dark and snowing. Our flight had been delayed by 4 hours and one of our bags did not make it (with a rehearsal dinner to attend the next day). We were not in our best form. We booked with Amex Platinum. The front desk person said "good news you have an upgrade to our executive suite" (we reserved the "one bedroom suite"). We got to the room which was probably the worst room in that executive suite category--on the ground floor, no view, semi underground and windows to the street so no privacy. This was a ski trip and we were also attending a wedding. We were very busy during our time there but I found some time at the end to check out why the heck our room was so not what we expected given the upgrade. Turns out we were actually downgraded--the room we booked was 1017 sq ft (we got 842 sq ft) and the room we booked goes for $700 per night more than the one they supposedly upgraded us to. Not to mention it was probably the worst room in that executive suite category given the layout of the hotel. Should have complained earlier but we had too much winter stuff and ski stuff, too many events, and skiing with kids to organize and the hotel was probably full.

We will probably never go back to Vail (too far and not worth it) and given past issues with FS room shenanigans and used car salesman approaches at the front desks in Maui, Cabo, and Santa Barbara over the years may be done with the brand. But how would you approach this to get maximum compensation?

Edit: They called us back, very apologetic, said they have talked to their staff about not letting this happen again and are refunding the difference plus a decent food and beverage comp. I will say that gym, housekeeping, spa and room service were lovely.