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The Discover it® Miles card is different from the United credit card in pricing terms and in the way rewards program works. The United card will allow you to earn 2 miles on United ticket purchases and only one and a half mile on all other purchases. So, if you are not going to fly before the trip you are planning, you may not be able to earn many miles. The Discover it® Miles card will allow you to earn 1.5 miles per dollar spent on any purchase. It means you can do your everyday shopping and earn miles toward your future travel.
Another thing to consider is the way you can redeem earned miles. The United card will allow you to redeem with United and its partners only, if there are available tickets of course. So, it may come that you won't be able to buy award tickets for all and at once. The Discover it® Miles card won't restrict you to which airline, when and how many tickets you can buy.
Also, don't forget about fees. The United comes with $525 annual fee which is not waived for the first year. The Discover it® Miles card has $0 annual fee.