Pretax 401k and Roth 401k- additional take home pay not factored in, is it?

Only if your marginal tax rate ("tax bracket") at retirement is less than your tax rate when you contribute. They are equivalent if the tax rates are the same. Here's the math:

Tc = marginal tax rate at time of contribution
Tr = marginal tax rate at retirement
R  = annual return on investment (same in either scenario)
C  = after-tax contribution amount

* For both scenarios, contributions are made exactly 1 year before they are withdrawn (ignoring the 5-year Roth rule just to make the math easier)

Scenario 1: Invest in 401(k) and tax savings in Roth:

                     401(k)     + Roth
                     ----------   -------
Initial investment:  C          + Tc*C
After 1 year:        R*C*(1-Tr) + R*Tc*C        = R*C*(1-Tr+Tc) 

Scenario 2: Invest everything in Roth:

                     401(k)           + Roth
                     ----------------   -------
Initial investment:  0                + C
After 1 year:        0                + R*C     = R*C

So if Tc = Tr, the two are equivalent. If Tr > Tc, then the Roth is a better choice since you're paying a higher tax rate when withdrawing the 401(k) then you caved when contributing (and vice-versa).

If you able to max out your contribution, then the Roth becomes more attractive, because you are able to effectively contribute more after tax (20k in a 401(k) if worth less after-tax than 20k in a Roth 401(k)). But if you max out the 401(k) and invest the tax savings in a Roth (assuming you are eligible to do so), then they become equivalent again (modulo differences in tax rates).

There are other differences, like the required distribution on 401(k)s, that may influence one's decision as well.