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The United Explorer Card has a $95 annual fee, which is waived for the first year though. Such annual fee is pretty standard for travel rewards credit cards. The other card, the United Club Infinite Card has a $525 annual fee. Such high annual fee implies access to travel perks and benefits like free checked bags, priority check-in and boarding and other travel benefits.
If you would like to have a travel credit card without annual fee at all, try the Discover it® Miles card. The annual fee for this card is $0, APR on purchases is 0% for 15 months (when the intoductory period is over, ongoing APR will apply), and there are unlimited 1.5x miles on all purchases. All the miles you've earned at the end of your first year will be matched if you are a new card member. For example, if you earn 35,000 Miles, you get 70,000 Miles.