I am not tall by any mean, 5'6" so typically legroom isn't an issue as is the seat size but had two varying experiences on a budget airline and one of the big name airline.
United Airlines - Economy - 737 BoeingCons: Last October, I took a direct flight on United Airlines flight from LAX to BOS on a 737 and squeezed into the economy seats. On both the outbound and return flights my knees nearly touched the front seat and the poor fellow next to me was at least 6'3" in the middle seat who put on a contortion act for us. Even worst, United allows these seats to recline just the slightest creating even less space between the seat in front.
Pros: United had LCD entertainment monitors with content you can purchase during the 3-4 hour flight
Spirit Airlines - Economy - A320 Airbus
Cons: Just last week I took a direct flight on Spirit Airlines from LAX to LAS on an A320 Airbus. The only downside I can think of is that their seats did not have any entertainment centers in the seats. Being a very short flight it wasn't necessary.
Pros: I had ample legroom, at least 2 to 3 inches compared to the longer flight to Boston on United Airlines. The price of the ticket more than reasonable while the seats were wide enough and had ample leg room even at rows 27 outbound and 20 inbound.
I thought I would hate flying on a budget airline such as Spirit Airlines. However, Spirit Airline has really keyed in their product with a young fleet of jets and exceptional pricing for the bare flight. For short flights I would absolutely use them again. I have a hard time saying the same about United Airlines economy. One thing they do have in common is that it's easy to add on extras such as booking larger seats (cheaper on Spirit typically) and paying for carryons or checked luggage as well as snacks on both flights.