Your taxable income is the 50k. Roth contributions are after-tax. Everyone needs to plug their own numbers, 3.5k in a Roth 401k may indeed be worth more than 5k in traditional when you retire, depending on the rest of your situation.
Usually it makes sense to go with traditional once you are in the 25% bracket or higher, and some states have different tax rules for 401ks. I live in New York state, which has a fairly high state income tax, but in retirement the first 20k that comes out of a 401k is exempt, so traditional 401k contributions save me money on state income taxes in this case.
Roth 401ks usually only make sense if you are in a low tax bracket now, are in an income tax-free state right now and plan on relocating in retirement, or plan on having a DB pension that will fill in your deductions/exemptions/lower brackets in retirement.
The best strategy in my opinion is to max out a traditional 401k and max out a Roth IRA. If you want further tax diversification maybe allot a portion of your 401k savings to the Roth option but leave the lion's share in traditional.