I've always seen that long-term cap gains tax rates have been lower than ordinary income tax rates, so I would vote for taxable account and not the non-deductible traditional IRA.
Some folks do the non-deductible tIRA so that they can convert to a Roth in 2010. The idea is that the conversion will cost them almost no taxes because only the gains (what are those?
) will get taxed at their high marginal income tax rate.
That written, I have an old non-deductible tIRA from when I thought I could do short-term trading essentially tax-free. I gave up on trading and just made that IRA 100% fixed income (TIPS and GNMA). Nevertheless, that prevents me from doing the 2010 conversion thing.
So I max out my 401(k) contributions, my spouse maxes her 401(k) contributions, some goes to 529 plans and the rest goes to a taxable account. With the taxable account, at least I can do tax loss harvesting. Those will keep my taxable gains tax-free for many centuries to come.