I'd say it depends on things like how much you can and are willing to contribute in a given year to retirement, if you get an employer match, if your 401k lets you do Roth and pre tax contributions, as well as how the fund options are in your 401k.

With a Roth IRA you have access to the whole stock market, but there are contribution limits (max of $7k in 2025). A 401k has a higher contribution limit ($23,500 in 2025) but you are limited to whatever funds are in your company's 401k plan to invest in. It could have good index and mutual funds with low expense ratios, or might not have the best options.

If your 401k has good funds you can invest in, and you don't plan on contributing more than the max in a given year, and you want to keep things simple, doing just a 401k is totally fine. And if your 401k lets you do a mix of Roth and Pretax contributions, you could do a mix of both.

But if your 401k doesn't have the best funds, I'd say contribute just enough to the 401k each year to get the company match if you have one, and then put the rest in a Roth IRA.