Diner's charges juice on airline conversions, but they have many more programs than Amex does. Considering most people use DC to top off accounts for awards, the juice isn't really that big of an issue. (95 cents or an extra 190 DC points per 1000 FF miles) It's nice to be able to move a needed three or four thousands miles into an account, and the $4 fee (while annoying) doesn't really bother me too much.

My biggest problem with Diners is that customer service in general appears to be slightly deteriorating. It's still the best in the biz by far, but they are doing little things that are putting them on a slippery slope towards becoming just another card.

Above and beyond all of this though: your company will probably choose the one that is accepted in more of the places your business will need to use it. If that is purely travel, it's a push. But Amex has the big advantage here when you get outside of hotels, rental cars, and flights.