Shopping for airfare is tough. Between hidden fees, basic versus main economy and dozens of airlines, each with its own terms and rules, it’s hard to know where to start. And one of the biggest questions is also the hardest to answer: When does it make sense to book with a budget airline, such as Spirit or Frontier, and when is it better to stick with a traditional carrier, like Delta or American?

Here we take a simple, step-by-step approach to answering this question. In the end, the question boils down to two factors:

How many amenities (including bags) are you willing to forgo?

How frustrated do you get navigating add-on fees?

For those savvy travelers willing to jet off in Spartan conditions, budget airlines can offer a very cheap way to get around. For others, it’s a safer (and simpler) bet to stick with traditional airlines.

Note: Most of the examples here are for U.S.-based airlines, for domestic flights.

What you need to know

How they differ

It’s difficult to distinguish precisely between low-cost and traditional airlines, but for the most part, budget airlines are those that add more, complex fees. Traditional airlines include basic services, like carry-on bags and in-flight soft drinks, for free.

U.S. airlines break down like this:

Traditional, full-service airlines:

Alaska Airlines.

American Airlines.

Delta Air Lines.

Hawaiian Airlines.

JetBlue Airlines.

Southwest Airlines.

United Airlines.

Budget, low-cost carriers:

Allegiant Airlines.

Frontier Airlines.

Spirit Airlines.

Sun Country Airlines.

(Technically, Southwest is a low-cost carrier, but by our definition, it doesn’t belong in the budget bucket because it charges far fewer fees than even most traditional airlines.)

How base prices can fool you

This is how budget airlines work: They offer extremely low base fares, which are what show up in search results. Then they tack on tons of fees during the checkout process. For example, Spirit Airlines often has the lowest base price for routes shown in Google Flights, but that doesn't mean it has the lowest price overall.

Comparing the real prices requires a bit of digital elbow grease. Here are the basic steps:

Use a search tool like Priceline or Google Flights to find several options.

Go through the process of checking out for at least two promising options.

Before you enter your credit card number, check the final price.

Comparing prices this way might sound basic, but it’s a hugely important step, and one that many travelers skip. If you're considering flying on a low-cost airline, make sure to compare the final price with that from a traditional airline.

Choose low-cost carriers when: You travel (very) simply

There’s one way to take advantage of the low base fares offered by budget airlines: Don’t pay the add-on fees. That might sound straightforward or easy, but it’s anything but.

For example, Spirit Airlines charges extra fees for:

Checked bags.

Carry-on bags.

Paying for these bags at the airport.

Drinks, including water.

Printing a boarding pass.

All seat assignments.

Be aware that most of these fees are charged each way, meaning the total fees associated with a budget airline flight can cost far more than the base fare, unless you’re careful.

The only way to truly take advantage of most budget airlines is to beat them at their own game, and that means adding few to no add-ons, including baggage.

Choose full-service airlines when: You prefer simplicity and flexibility

Dealing with budget airlines’ convoluted fee structures includes a cost that can’t be summarized in a table or calculator because it’s psychological. Trying to navigate these fees and “bundles” requires a lot of expertise or patience, and many travelers can’t be bothered.

That’s not to say that traditional airlines don’t have complex add-on fees. Many do. But they're generally slightly more transparent and easier to understand than the downright misleading fees charged by budget airlines.

If you’re planning to bring a carry-on bag, select a seat or enjoy an in-flight soft drink, the cost difference between budget and traditional airlines could be negligible. And if you’re not completely confident in your travel dates, most traditional domestic airlines now offer greater flexibility when it comes to changing and canceling your flight.

If you’re on the fence: Consider basic economy

Adding one more twist to this maze, traditional airlines have added low-cost “basic economy” fares in an effort to compete with budget airlines. These fares generally differ from “main cabin” fares in a few ways:

In general, you should book basic economy when you're traveling alone with firm dates and are not a frequent flyer.

The bottom line

Comparing airlines can feel overwhelming, and it’s easy to throw your hands up and book the lowest base fare. But that’s exactly what you shouldn’t do.

Learn the differences between budget and traditional airlines, compare total costs and err on the side of simplicity by choosing a traditional airline. If a fare looks too good to be true, it usually is.