You need a professional financial planner.
If you invest solely in after-tax stocks, mutual funds etc., you will have less money to invest ($1000 pre tax vs $700 after tax, for example). And, as your investment grows, you will pay tax each year on dividends, and realized capital gains in the funds. Then when you retire, most of the money you withdraw will be a mixture of original cost basis (no tax) and gains (15%).
Money in a tax-deferred retirement account is taxed as ordinary income, which might be 12% or 22% under current laws depending on how much you take out. 22% is certainly higher than 15%. But, you invest more each year (because it's pre-tax, you can afford to put more in) and the growth is also tax-deferred.
Which is better for you depends on too many factors for us to tell you what to do.
(And this ignores the Roth IRA, and investing in tax-free muni funds, which adds another layer of complexity.)
