Take a look at the investment order in this post:
https://forum.mrmoneymustache.com/investor-alley/investment-order/The tax benefits of the 401k/IRA/HSA are substantial and in most cases you should max them out before investing in taxable accounts. If you're saving large amounts of your salary you'll probably exceed these limits anyway, and then you can plunk the remainder into a taxable account. Pick tax efficient vehicles (i.e., index funds that don't realize lots of transactions and capital gains, perhaps tax exempt bonds if your investment strategy calls for these) for the taxable account.
Note that you can tap your Roth contributions at any time without penalty anyway, as they were made after-tax. Whether you should pick conventional or Roth is another matter. It basically boils down to whether you think your tax rate now will be higher or lower than your tax rate in retirement. If you're living on a small amount now and expect to do the same in retirement, you may want to bet on "lower in retirement" and contribute to a traditional (i.e. tax deferred) 401k/IRA.
One more factor: if you are able to max out your 401k/IRA and contribute additional funds to a taxable account, contributing to a traditional account in effect gives you more money to stash in taxable accounts (due to the tax break now) and then you have both "buckets" to diversify your tax strategy later.
See the Mad FIentist and GoCurryCracker blogs for lots more on these topics.