As you're aware, a 457 plan is a non-qualified retirement plan for public-sector employees.
If you are withdrawing money from the 457 plan you will be able to take penalty-free withdrawals. If the funds are contributed or rolled over to the Roth, however, it would be taxed as ordinary income. The Roth IRA would be valuable for tax-free growth and withdrawals but you do need to consider the taxation of the 457 monies. Make sure to discusss this with your tax advisor.