A mega backdoor Roth involves putting money into an “after tax” (not Roth) 401k account and then rolling it over into Roth either within the 401k plan or to an external Roth IRA. It can mean an extra $25k+ in Roth on top of what you’re already contributing to the 401k. Most plans do not offer the features necessary to do this.

Assuming tax policy stays the same, you’d probably lean traditional if you expect your income to be lower in retirement but you’d also want to consider tax diversification. Another aspect is that Roth IRAs will allow you to take contributions back out tax and penalty free before retirement which can be useful if you planned on retiring early.