Generally speaking, travel-rewards credit cards impose annual fees ranging from $95 to $550. Sometimes the fees are waived for the first year, sometimes not. But long term, the guiding principle has been that cardholders should expect to pay for the extra value that the cards deliver over no-annual-fee non-rewards credit cards.

Recently, however, two of the largest airlines, Delta and United, and their card issuers have rolled out rewards cards that have no annual fee—not for the first year, not ever. Both are entry-level cards, stripped of the enormous sign-up bonuses and long lists of travel perks that come bundled with more expensive cards. But the cards are more different than they are similar.

Here's how they compare.

Blue Delta SkyMiles® Credit Card from American Express

Welcome offer: 10,000 bonus Delta SkyMiles® after spending $500 within the first three months of account opening..

Annual fee: $0. 

Earning rate: Two miles per $1 spent for dining at U.S. restaurants and purchases direct with Delta; one mile per $1 spent for other charges.

Comment: With the exception of a 20 percent discount on inflight purchases, the Blue Delta SkyMiles® Credit Card from American Express is perk-free.

United TravelBank Card

Welcome offer: $150 in TravelBank cash after spending $1,000 within the first three months of account opening.

Annual fee: $0.

Earning rate: Two percent on United tickets; 1.5 percent for other charges.

Comment: Rather than frequent-flyer miles, the United TravelBank Card awards TravelBank cash, which can be used to purchase United tickets, either alone or in conjunction with other forms of payment. There's a 25 percent statement credit on food and beverages purchased inflight with the card, and there are no foreign transaction fees.

Blue Delta SkyMiles® Credit Card from American Express vs. United TravelBank Card: Which Card Is Right for You?

There are two key differences between the Blue Delta SkyMiles® Credit Card from American Express and the United TravelBank Card, both of which target younger, less affluent, less frequent travelers.

First, there's the difference in airline affiliation. If you're a Delta loyalist, there's probably no point in considering a card from United, no matter how rewarding it might be. And vice versa. When evaluating airline travel-rewards cards, always begin by choosing the airline that best meets your needs; the choice of a rewards credit card should be secondary.

The other significant difference is between the two cards' rewards currencies: SkyMiles in the case of the Blue Delta SkyMiles® Credit Card from American Express, TravelBank cash in the case of the United TravelBank Card.

Of the two currencies, miles offer by far the most flexibility. They can be redeemed not just for flights on Delta and its airline partners, but for a wide range of other products and services. By contrast, TravelBank cash can only be used to purchase United flights.

On the other hand, the 1.5 percent earning rate for TravelBank cash is slightly more generous than the one-mile-per-$1 earning rate with the Delta card, if Delta miles are valued at 1 cent apiece.

But again, the real decision driver should be which airline is the best fit with your travel patterns, rather than the particulars of either of these cards. And then the more important decision becomes not which of these cards to apply for, but which Delta card, or which United card to apply for, since the new no-fee cards are among several rewards cards offered by both airlines.

Delta and United both offer mid-priced cards, with a $95 annual fee, and high-end cards, with a $450 fee. They're more expensive, to be sure. But if you can take advantage of the pricier cards' sign-up bonuses and travel perks, they might represent better value than the no-fee/no-perks cards. Sometimes you do get what you pay for.

Editor's Note: This content is not provided by any bank, credit card issuer, airline, or hotel chain. Any opinions, analyses, reviews or recommendations expressed here are those of the author's alone, and have not been reviewed, approved, or otherwise endorsed by the aforementioned entities.