A good FSPP can negotiate with the hotel before booking to see if upgrades can be secured in advance if that's important to you. There is definitely an advantage to using a FSPP to AMEX FHR. With AMEX you get perks because of the contract with FS, but there is no personal connection to the hotel and the people answering the phone at Amex have probably never met a FS representative. A FSPP usually has a personal connection with at least the FS, if not that specific property.
Just as an example last month I was in Maui and had a 1:2 dinner with both the Sales Managers for all the FS Hawaii properties. That's a connection that AMEX can't offer. Feel free to PM me if you want more info or you want to work together.