Both funds are dependent on longevity with the airline, and both funds will pay out more with more years, but for different reasons.
The A fund is determined by some sort of formula that involves longevity, highest pay earned, and life expectancy. The main variable at different airlines is how the highest pay earned is figured. At my airline, its the highest 60 months (5 years) - so it obviously pays to go widebody sometime in your last 5 years. Your A fund will pay you more if you fly as a 777 Captain your last 5 years than if you fly as a 737 Captain. In order to get to be a 777 CA you have to be with the company a long time, obviously.
The B funds rewards longevity with the beauty of compound interest. Ours is 11% of pre-tax earnings, put into a fund managed by an independent company. If you want to manage your own money, you can do the 401(k), subject to federal limits. The beauty of the B fund is this (all done with a calculator by hand on a long, boring San Salvador turn): With conservative estimates of static wages at today's rates and 8% annual growth, by year 10 the growth of your B fund outpaces company contributions. The difference becomes larger as the years go on, until by year 25 the growth is $100,000/year, while company contributions are still 11% of your salary. The longer this money grows, the more you will have at the end.
The same goes for the 401(k). The money goes in pre-tax, and grows tax-free. A few thousand more dollars now will be worth more than almost anything you can shovel at it when you're 55. If you can afford to max out your 401(k), do it as soon as possible. You'll be amazed at the results.