What is the difference between the retirement options?
…is what is known as a defined benefit plan. That is a retirement account that is funded by your employer and promises you a set payout when you retire.
A pension happens automatically per your union’s contract with the employer.
….is a defined contribution plan, meaning that you contribute your own money (pre-tax) and it grows until you retire, at which time you have access to whatever it is worth.
You must sign up for and then actively contribute to the 401(k) plan in order for it to happen. (Your employer may also match funds, depending on the contract.)