There are many things to consider when choosing an airline. Most people just go with price, which is understandable considering the costs of travel these days. But where you frequently fly from and to are just as important because you want to focus all of your frequent flier miles into one airline's program to get the most benefit. Customer service is another big factor, as is the amount of destinations served and the type of aircraft used.

Frontier flies to Mexico, Canada and Central America, while Southwest is limited to domestic travel only. However, if you don't plan on flying out of the country, Southwest airlines does have more options for US cities and states.

The average price listed in the features section is for a round-trip flight between San Francisco and Washington DC booked and paid for one month in advance. Flight costs will vary, but we found Frontier to be slightly less expensive on most trips with the exception of to and from Dallas and Las Vegas.

Customer reviews online generally give both airlines between 3 and 4 stars out of five, with Southwest having a slight lead over Frontier. The service is nothing to brag about on either airline, but what can you expect for a couple of hundred to cross the entire continent twice.

Upon enrolling in Frontier's program (Early Returns) members start at the first level, Base, and accrue miles as they fly with Frontier or their partner airline AirTran. Once a member gets to 15,000 miles in a calendar year, they reach the Ascent level and receive free DIRECTV service, priority boarding and ticketing, priority seat assignments, and a 25% mileage bonus in addition to miles accrued. The third level is called Summit, and is reached at 25,000 miles in a calendar year. Summit members enjoy a 50% mileage bonus in addition to miles accrued on each flight. In addition to all of the benefits received by Ascent members, Summit members also get complimentary alcoholic beverages and more mileage.

Upon enrolling in Southwest's frequent flier program (Rapid Rewards) travelers receive one credit for each one-way trip. A free ticket, expiring after 11 months, is automatically issued when a member accrues 16 credits in 24 months. In addition, one-half credit is also earned for using a Southwest partner to book any car rental and/or hotel stay, regardless of whether a Southwest flight is involved. Southwest limits the seats offered to frequent travelers using free certificates on each flight, whereas previously if there was a seat available, one could use the award, provided the passenger was not flying on one of the five blackout dates. In early 2006, Southwest expanded its codeshare agreement with ATA Airlines and now allows redemption of award tickets on Hawaii flights at the rate of two awards per round trip flight.

These are the two best low-cost airlines operating in the United States today. Service isn't spectacular, but you can get just about anywhere you need to go in the US on either airline, and for a much lower price than many other airlines. In the end, Frontier receives the Editor's Pick badge because their prices were lower, they fly outside of the United States, and we liked the rewards program better.