General retirement advice is to contribute 15% total to your retirement account. Since your employer adds 12%, you should contribute a minimum of 3%.

As for your coworkers advice, it may or may not be valid. Contributing to your 401k or a taxable account, both allow you to invest your money. The 401k comes along with tax advantages. Assuming you are making Traditional contributions, then you are saving taxes at the marginal tax bracket (potentially 22 or 24%). If you contribute enough to drop your MAGI to the 12% bracket, then the tax benefit is much less. You can still contribute to Roth, which means you can withdraw that money tax free in the future but you have to pay taxes on it now.

Another thing to consider is the fees in your 401k. Since you are limited to the options provided, sometimes fees are quite high. If you don't have any options that cost less than 1%, you likely are better off investing your money elsewhere.

Finally, you get into the really personal part of personal finance. What are your goals? Do you have short/medium term goals that you would prefer to save this money for? For instance, if you want to save up for a down payment for a house in the future, putting that money in a 401k is a bad choice. Do you have a comfortable emergency fund to pay for unexpected expenses? Do you have high interest debt to pay off?

A good resource is this flowchart from r/personal finance: https://imgur.com/u0ocDRI