Why not do both? For now, take the AA card with its 12k bonus miles and 6 months free. Use it for 5 months or so, and then get the Diner's Club. By then, you'll hopefully be able to use Diner's everywhere that MC is accepted. You'll be able to earn 12k AA + 24k DC in sign-up bonuses.

Pros for DC: car rental insurance, largest/most flexible program in terms of air/hotel partners, exceptional customer service, and - by midyear - accepted almost everywhere.

Cons for DC: annual fee, slightly-reduced earning power ($1 = 0.913 FF miles).

Your volume will dwarf the statistical impact of the annual fee. The flexibility of DC is worth the slight hit on earnings, IMHO.