I was in both lounges a few days ago, and thought the breakfast offerings were fairly similar and quite acceptable. However, on the drinks front, I was appalled by the service at the bar in the Amex lounge. I asked for a sparkling white wine to go with my breakfast and was met with a stony look and was served a thimble full (I guess the barperson disapproved).
So I ate my breakfast and moved to the House, where I was warmly welcomed and served whatever drinks I requested with a smile and in decent measures. Won't be visiting the Amex lounge in a hurry again...