American Express Platinum = Membership Rewards
Merrill+ = British Airways
Thanks. I am not very familiar with either of these. But I suspect that the original poster would get more miles per dollar spent on most airlines except United and Continental by using the Starwood American Express card, as previously mentioned, if miles are what is sought. Info on that card and its up front bonus is of course available on the Free Cards page of the Credit Card section of my website below.