With credit cards being such a huge source of miles and points for travel, the question of which one is best to use is often a shifting field. The cards in my wallet have changed a bit in the past few months.
Amex - Everyday Preferred
This may not be the most obvious card, but it earns Amex Membership rewards points. Membership rewards points are like Citi ThankYou Points and Chase Ultimate Rewards points. They are transferable to many different airlines. The big one with Amex points is Delta, where I'd have to say membership rewards points aren't as good with the other transferrable partners since many of them don't transfer 1 to 1. Why is this my most used card? After 30 purchases, you earn an additional 50% on Membership Rewards points, even on the Grocery Store bonus category. If you meet this threshold, you get 1.5 points per dollar on everyday purchases, and 4.5 points on grocery store purchases. This little bonus makes quick work of building your points balance.
Keep in mind one little factor. Most other cards issue the points earned upon the end of the billing period. This card works differently and issues the points after the next billing cycle, so instead of a 1 month lag, it can be up to a 2 month lag from purchase to account.
Citi - ThankYou Premier
Bonus categories on the ThankYou Premier card are as good as it gets. 3x points on gas station purchases and travel. Since travel is a bit light at the moment in my life, this one is being used almost solely for gas station purchases to grab the major bonus here.
Chase - Sapphire Preferred
Chase's Sapphire Preferred card is a great all around card. It isn't the best in bonus categories, but also not the worst. To round out my spending, this card is used for restaurants in order to grab the 2x bonus on this type of spending. While the Citi equivalent gives the same bonus, using the Sapphire card here helps diversify my points into the Chase arena.
Keep in mind that this credit card acts as primary insurance for any rental car that you purchse with this credit card. That means no points on your license for an accident, since your normal car insurance is not used first for any accidents, this credit card is the primary insurance on your rental. This little detail is huge for rental cars. While I hope to never use this feature, it's a nice peace of mind.
Right now the Amex Everyday Preferred card is the major everyday spend card for me. As my membership rewards points increase, I'll likely shift back to my Amex Starwood Card since Starwood points are worth so much and are more diverse than any other point system out there. Starwood can transfer points to almost any airline you can think of and that may be worth losing the bonus 0.5 points on the Amex. This is something I still need to look at with the general worth of the two different types of points to see how to best manage my everyday spending and earning.