When should you contribute to 401k Roth instead of 401k Traditional? Are there any rule of thumbs which relate tax bracket to this decision? Assume that one makes a contribution towards retirement in taxable of atleast the same amount as the tax saved on 401k T.

Fox example, say the marginal tax rate is 30%. Mr. X contributed 17,500. He also contributes 5250 in his taxable account towards his retirement. Now which would be better (a) or (b):

(a): 17,500+5250 invested for 30 years with returns on 17,500 as ordinary income and returns on 5250 as long term cap gains.

(b): 17500 growing tax free.

We need the rate of return, time horizon, tax rate at withdrawal to crunch real numbers here. Curious to know if there is any already defined wisdom on this already.