If you do not intend to contribute more than 25k into Roth accounts, the Roth 401k would be an option for the first 19k. If you are investing primarily in mutual funds, many of the funds within the Delta Plan have lower fees than their counterparts outside of the plan. You can roll the Roth 401k over to a Roth IRA at retirement to avoid the RMDs. The Roth IRA must be open for 5 years before taking funds out, so you could start a backdoor Roth IRA of $6k per year in addition to the Delta Plan and then roll over your Roth 401k at retirement. The rollover funds would pass the 5 year test if the Roth IRA has been open for 5 years.

DYODD, YMMV, get some real advice or at least spend some time on google.