That's a nice choice to have. Are you sure that the points program is included with both cards at no extra fee, or is there a fee you have to pay to include the points programs?
IMHO, MR is too restrictive, Diners is more flexible, Diners usually has a 1:1 promo at some point during the year, but Diners charges a small fee for airline miles redemption. Diners recently increased their membership fees, but that doesn't affect you as a corporate cardholder.
As far as service is concerned, hands down, DC wins there. Though there's been a few recent complaints from DC members, but nothing of the level of complaints you'll see in the Amex forums, which show a long pattern of bad customer service and treatment. It's too early to tell if the DC complaints are a trend or not. Personally, I've had nothing but great service from DC when I've called them.
I still think DC is better, though some people left DC when the mambership fees were recently increased (but as a corp card holder, this probably doesn't affect you) and they added the redemption fee for airline miles.
-David
[This message has been edited by LIH Prem (edited 11-23-2002).]