Other items to keep in mind:

1) Amex cards do not have access to PP Restaurants that all other PP memberships through US-issued CCs come with. Domestically the biggest one would be PDX which has multiple restaurants but no Amex option unless you are flying DL, and there are some others out there that have PP restaurant access but no lounges through Amex without going through security twice like SFO T2.

2) Some of the PP lounge options are really mediocre compared to the Amex option(s). JFK T4 comes to mind as an example.