This is a complicated questions with no single easy answer. People save in all kinds of different places for different reasons.
The key is making sure you have access to your money when you expect to need it. For some people, this means only investing in taxable accounts or real estate, which have no restrictions. For others, it means taking full advantage of tax-advantaged retirement accounts like a 401k plan and then finding ways to work around the age-based withdrawal restrictions. I fall in the latter camp.
For an early retiree who will be spending much less in retirement than he currently earns, the tax benefits of a 401k are enormous. You 100% avoid paying taxes now while your income is high, and then you instead pay taxes later when your spending is lower, so you pay a much smaller amount. In some cases, you can end up never paying taxes at all if your deductions are greater than your spending, which is pretty easy to do with a family.
One common plan for utilizing a 401k while still retiring early is the 5 year pipeline rollover method. You need to have five years of retirement spending saved up in non-restricted accounts like a savings account, taxable investment account, or Roth IRA (contributions only, not earnings since those are restricted). Then the plan goes like this.
1. Retire, dropping down to a much lower tax bracket.
2. Convert one year's worth of your lower expenses from your 401k to your Roth IRA, paying the taxes immediately.
3. Repeat that conversion every year for five years while living off of your non-restricted savings.
4. After the first year's rollover conversion from your 401k to your Roth has seasoned for 5 years, it counts as principal and can be withdrawn penalty free and tax free from the Roth IRA.
5. Every year you roll over another year's expenses for five years down the road, while withdrawing one year's expenses from the Roth from 5 years ago.
This system requires you like doing paperwork or have a good accountant, but it allows you to access your 401k money tax free and penalty free at any age. Depending on your 401k rules, you may have to convert from 401k to traditional IRA to Roth IRA instead of converting straight to Roth.
Summary: I suggest maxing your savings to your 401k or other tax-advantaged accounts as long as you can also manage to put away 5 years of expenses in non-restricted accounts.