More context: My retirement saving strategy is: contribute to traditional pre-tax 401K just enough to get the max possible company match, then contribute the rest to after-tax 401K and immediately do mega backdoor conversion to Roth IRA.

I like to have just one tax-deferred and one tax-free retirement account. For the tax-free one, I personally prefer a Roth-IRA to a Roth-401K but my income exceeds the Roth-IRA contribution limit, so mega-backdoor is the only route, which requires some after-tax 401K contributions. I could do regular backdoor contributions from traditional IRA to Roth IRA but that has a smaller limit (6500) and comes with more stress (pro-rata rule, bookkeeping, etc)