Another Doc here. The question is confusing. A 457(b) is a deferred compensation plan.
Generally, putting tax deferred money into the 457 is preferable at or above the 28% federal income tax bracket. If in the 25% bracket or below, a Roth 457 contribution is probably the better option.
The Fidelity Roth may not be an option if their combined income in 2016 exceeds the limits for contribution (~180,000).
Hope that helps.