I also just got the Sapphire and have been wondering about the same thing. I guess two big questions for me are how much I value Amex customer service, and how much the Platinum perks are worth the $250 (to me, I can use the airline allowance).
That said, since my main interest are premium class airline redemptions, if they don't add United after the merger it will be a no brainer to stick with Amex - since BA miles are practically useless at this point, and by many accounts probably getting worse.