Right, you are either taxed on the funding side (Roth), or the withdrawal side (401k or traditional IRA). If the tax rate is the same in both instances, then the net proceeds are identical regardless of the the rate of return on the account. If the tax rates are different, you are better off biasing to the lower tax option.
I've heard this multiple times...
When you put numbers to it in the generic sense, it all balances out
For instance:
I put in $10,000 post tax dollars into my Roth 401-K and bought 100 shares of TSLA when it was $100
TSLA grows to $10,000 a share
If I am in a 33% tax bracket, that means I paid $5000 on my shares upfront and now I have $1,000,000 in TSLA
If I put in the $15,000 investment (counting for the upfront 5K in taxes) then the numbers are similar to the first option - shares worth 1M after taxes
HOWEVER,
If you are able to put in the max contribution for your 401-K (it is now 19K, but lets use the $10K to make the numbers easier), it makes a huge difference if you have a Roth vs non ROTH option. You can put in 10K in either the Roth or non Roth option, then clearly the advantage is in the Roth option
Think of it like this...
I use the same $10,000 investment into a post or pre-tax shelter account purchase my shares of TSLA
In my Roth 401-K, TSLA goes to $10,000/share so I net 990,000
In the pretax 401-k, and TSLA does the same ($10,000 share) now I am paying taxes on 1M, so now I get a net of $666,666
The 2 best reasons for CONVERTING a preexisting 401-k to Roth
1. you feel the tax rate is going to go higher or you will be in a higher tax bracket when you retire
2. the psychological benefit of not having to worry about paying taxes when your income drops in retirement