Personally, I would always choose the Amstel over the CP.

I would email them both before booking, ask if it's possible to add a third person and see what they say. You might get more flexibility with the CP than the Amstel because of your IHG status.

You could also try asking the Amstel if you were to buy Ambassador status through them, would they upgrade you to a room that sleeps 3 people. I think they get commision for selling ambassador status.