The advantage Diners Club has over all takers is that the points are transferrable to all the major hotel programs (something not even AMEX can claim) as well as airlines. If you're just looking for points in one chain that can be converted to different airlines, the Starwood card may be the way to go, but for the most flexible awards program, I don't think anyone can argue that Diners Club is it. Just remember, the most valuable airline miles/hotel points are the last 1,000 you need to redeem a specific award ...

Folks can argue the Diners Club card is expensive (which I personally don't believe), but to say it's expensive requires there to be a point of comparison, and no program can compare to the flexibility of the Diners Club program. So, while other cards are less expensive, they also do not offer what Diners Club offers (you can argue that the premium may not be worth it, but that's another story).