401k: as ur money is pre-tax(larger amount), it grows faster(bigger base).

for example: if u have $3000 pre-tax money this year and you're 30, if you put it in 401k, it will grow from $3000 for 30 years(suppose u can use it at 60).

if you don't put it in 401k, but pay the tax(you may get only $1500) and invest it by yourself, then it will grow from $1500 for 30 years(and each year you have to pay tax if you have positive return on your investment), think about how different the amount will be.

there's more information on web