I would suggest the Chase Sapphire Preferred over any other card for your first, even over the Sapphire Reserve. I have several cards on my wallet, and the preferred was my first. Since you're starting out with a card, the CSP is way better than the CSR in terms of earning, annual fee, sign up bonus, and potential redemption.

Unless you're going to be doing a lot of paid flying with either airline, I don't think you should get one of their cards first.

Take a look at The Points Guy if you're interested in doing points and miles, that website really helped educate me when I got started.

I'll also echo what's already been said, depending on where you're based, and which airline is the dominate player there, and your destinations, that should drive your airline preference.

Also, know your credit score and check online for what the minimum is for various cards. Amex tends to be higher than others.