“Better” depends on what you mean. Base gasoline at Costco and ARCO often comes from the same West Coast supply system. The practical difference is the detergent additive treat and how you pay.
TOP TIER vs minimum detergent
U.S. Costco gasoline is licensed TOP TIER. That program requires a higher detergent dose than the EPA’s lowest additive standard. ARCO stations are generally not TOP TIER retailers. Several automakers recommend TOP TIER fuel to limit intake-valve and injector deposits.
That does not mean ARCO fuel is unsafe or off-spec. It means Costco’s U.S. gasoline is certified to a stricter deposit test. If you want that standard at a warehouse-club price, Costco is the closer match to Shell, Chevron, or other TOP TIER brands — see Costco fuel additives.
Price, membership, and locations
ARCO is often cheap and open to anyone (card policies vary). Costco requires a membership and is only at warehouses with fuel. For California drivers, start with Costco gas prices in California, including the Los Angeles warehouses.
If you already shop at Costco, the TOP TIER spec plus member pricing is usually the better regular-unleaded buy. If you are not a member and just need a gallon, ARCO may still win on convenience.