稅率不變的話，都是一樣的。

稅率：T_401K%   vs. T_ROTH%

回報：R%

401K Return =  principal * R%* (1-T_401K%)

ROTH return = principal * (1-T_ROTH%) * R% 

If T_401K% = T_ROTH% = T%

401K return = principal * R% * (1-T%) = principal * (1-T%) * R% = ROTH return