I have the following cards for the following reasons:
1. United Visa - I want to get to both my childhood home and Hawaii. United does this for me.
2. AMEX - wide acceptance and good list of partners.
3. Sheraton AMEX - got it as a gamble after I heard they were coming out with a new card. I don't know that I will keep it after the free year is up.
I hope that sharing some of my thinking helps out. There are a ton of them out there and if you're unsure my suggestion would be to start with a no fee card so you don't have a sunk cost when you figure out which one is really for you.