Next is the United Explorer Card.

Welcome bonus: Earn 40,000 United miles after you spend $2,000 in the first three months, and earn an additional 25,000 miles after you spend $10,000 total in purchases in the first six months.

Annual fee: $0 introductory annual fee the first year, then $95

Earning miles: 2 miles per dollar spent on United purchases and at restaurants and on hotel stays, 1 mile per dollar on everything else

Top benefits: Free checked bag, priority boarding, two complimentary one-time United Club lounge passes each year, up to a $100 application fee credit for TSA PreCheck or Global Entry, access to additional award space, no foreign transaction fees

The United Explorer Card from Chase is a solid option for United flyers. Like the other airlines' sub-$100 cards, the United card offers a free checked bag for the cardholder. Up to two traveling companions booking at the same time can also get a free checked bag each. This is especially useful since United raised the price of a checked bag to $30.

Unlike the other airlines' cards, though, you'll have to actually buy the tickets with the United card in order to get the free checked bags — simply having the card open and tied to your account isn't enough.

For those who don't have luggage to check, the card offers priority boarding for everyone on your reservation — you'll get this benefit even if you pay for the tickets with another card. By boarding earlier, you can make sure to snag overhead space for your carry-on.

One excellent perk with the United card is that you'll get two complimentary one-time passes to United Club lounges each year. If you get to the airport early or end up dealing with a delay, you can head to the lounge and enjoy free snacks, drinks, Wi-Fi, and more comfortable seating than what you'd find in the terminal.

When you spend $25,000 in a calendar year, you'll also get a PQD waiver — PQDs, or "Premier Qualifying Dollars," count toward earning elite status with the airline. When that requirement is waived, you'll earn status just based on how much you've flown, rather than a combination of how much you've flown and how much you've spent that year.

The card earns 2 miles per dollar spent on all United purchases, and it also earns 2x miles on all dining and hotel spending. It will continue earning 1x mile on everything else. There are no foreign transaction fees.

Other features include a fee credit to cover the cost of enrolling in Global Entry/TSA PreCheck, and a 25% discount on all in-flight United purchases.

One unique — but unpublished — perk of the card is that having it helps you access additional United award space. That means that you'll have an easier time finding the flights you want when it's time to actually use your miles.

Click here to learn more about the United Explorer card.