When the stock market tanks, companies are required (by ERISA) to kick in extra funds to maintain minimum required pension fund assets. When the pension fund investments do great, the company can slack off with their contributions to the pension fund. So, the company never quite knows what its pension expenses will be. They do supposedly have professionals managing the pension fund. On the other hand, if the company goes tits-up and no longer makes contributions, the pension you were promised goes to hell. These "defined benefit" plans all of a sudden are "defined unless something bad happened" plans. And you know that no one goes to jail when pension promises are broken. Ask TWA pilots what happened to their pensions.
401-K, and such (e.g. Keough) plan expenses are much more predictable as they relate to payroll and the stupidity level of employees who do not take advantage of employer matching contributions. That's why they call them "defined contribution" plans. The employee decides how the contributions are invested. Unfortunately, many employees are not that savvy with their choices. And, employers don't often provide the very best choices to begin with. But, the employee does make the investment choices and the company has no way to take that money back.
Pension plans usually have a vesting period, say 5 years. If for some you reason haven't worked long enough, you get zip!
So, do you want a bird in the hand or two in the bush?