@Albort brings up some really good points. Do you value cash back or points? You might also consider how close you are to Costco (or Sam's), whether you shop or fill up there frequently, or do you pump elsewhere? (But if you're only pumping reimbursable gas, your considerations might be different.)
The Costco Citi Visa is great because it offers 4% cash back at gas stations anywhere up to $7,000 spent per year. (This card also earns 3% on restaurants and travel, and 2% on Costco purchases.) So you can earn bonus cash back at Costco gas stations or anywhere else you pump, but keep in mind 4% of $7,000 is only $280, then you earn 1% cash back thereafter. (This could matter if you spend a lot on gas each year.) But considering Costco's gas prices can be considerably cheaper than nearby gas stations (my closest Costco gas station is usually $0.20/gallon cheaper than its nearest competitor), using Costco's cash back card can add up to a good amount of savings. While I agree in most cases, cash is king, you get the cash back in the form of a check once a year (iirc, around February). I don't know how you feel about waiting a year to get your cash back.
When I'm not pumping at Costco (because it can be kind of a drive), I use an Amex Premier Rewards Gold which earns 2x Membership Rewards at gas stations because I value flexible points like UR and MR about as much as cash. Amex has a targeted offer right now until 9/30/17 that earns an extra 4x on gas, which makes a huge bonus. Unfortunately, the offer seems highly targeted and I didn't get it.
Note that since Costco ended their relationship with Amex last year, you can no longer use Amex cards at Costco warehouses or gas stations. Costco accepts only Visa. Of course, you can use Costco's Citi Visa at non-Costco gas stations.
You might also consider whether your local grocery chains offer gas points. We shop at Safeway, which has a pretty low earning gas points program, but lets us offset gas at Chevron by up to $0.20/gallon up to 25 gallons. I know Kroger often runs 4x gas point promos on gift cards (including Disney gift cards) that the folks on the Disney Gift Cards thread really know how to maximize to lower their gas prices and offset Disney vacation costs. I'm not familiar with other chains or their promos.
I'd also add that if you're still under 5/24, you might want to delay getting one of these cards for just gas unless you find real value in the extra bonuses. The signup bonuses from Chase cards while under 5/24 are really lucrative, whether in UR points, SW and United miles, or hotel points, which are often worth hundreds of dollars. The Costco Citi Visa never has any good signup bonus -- I recently saw my local warehouse offering a free cooler bag for applying. I think Costco offered a free rotisserie chicken last year, and maybe a free 5 gallon empty bucket at one point. And it's benefit for gas is basically a bonus of up to $210 on top of regular 1x spending. The Amex PRG, however, is worth getting eventually for the signup bonus, which can be up to 50,000 MR points, regardless of what you end up using the card for. The 50k points is targeted, but people have gotten the offer using incognito/misspelled name and old address/Chinese search engine tricks.
Hope some of this helps.
