If your company shares in the after-tax sub-account are highly appreciated, the gains will be taxable if you roll the shares in-kind over to a Roth IRA and you would pay ordinary income tax on those gains. NUA treatment allows the gains to instead be taxed at long-term capital gains rates. Depending on your situation, the lower taxes on the gains might outweigh the benefit of future tax-free growth in the Roth IRA. The analysis is beyond what is practical to do via a forum like this one. You would be better off finding a tax advisor with experience in this area.
Perhaps you don't even qualify for distribution of NUA. To qualify for NUA you must make a lump-sum distribution from your 401(k), distributing everything in a single tax year with no distributions in the intervening years between the year of your qualifying event (generally age 59½ or separation from service from the company) and the year of the lump-sum distribution. Other parts could be rolled over to a traditional IRA to continue to defer taxes.
