If you don't have a specific goal in mind, the Sapphire Preferred is probably a good place to start. You aren't committed to an airline, so there's flexibility in what you use the points for. Also double rewards on dining/travel. Not sure what the best offer is right now...40k I think? Also getting the Amex SPG wouldn't be a bad idea...the cash+points option allows you to really extend the value, and you can always transfer the points to an airline if you aren't going to use them. They'll build up nicely if you're staying at SPG properties as well. [I think this is the only program that lets you transfer points to American, although I might be forgetting something?] And of course there's the Citi AA cards, which is a quick way to build up miles on AA but aren't the best for everyday use.

the cash back vs. rewards question is really up to you. For example, the Amex Blue Cash does 6% cashback on groceries, which is better than pretty much anything else. But, if you end up using rewards for premium international travel, the 'value' can often be much more than that. Me, I'm much more interested in traveling fun places than I am in getting a few hundred cash back, but that's my situation.

Anyway, if you decide to go for the Amex SPG card, send me a PM and I can give you a referral - 25k pts.