I should have mentioned earlier, although it's kinda obvious, that with a taxable account you pay ordinary income taxes on your cost basis before you even purchase the investments, just like the rest of the money in your paycheck. For instance, you could save $400/month in a taxable account or an extra $500/month in a 401k, and still have the same money left over from each paycheck in either case because the tax withholdings would make up the difference. While it looks like you're saving more using the 401k, don't forget that you're just delaying the taxes you'll eventually have to pay on the whole amount, which will probably be at least 15%.
Taxable investments get effectively taxed upfront at whatever your current marginal tax rate is (25%, 28%, etc). You also will get taxed on the gains and dividends over time as my previous post described, probably at a 15% rate as long as you're still bringing in a decent income. You can offset some of these taxes in the short term with tax loss harvesting, but this operates a lot like an IRA; you get a discount on your taxes this year in exchange for an increase in some future year.
That should be enough info to let you do the math and see the difference. I think starting a small taxable account of about $5,000 is a good idea to get familiar with how managing taxable assets works in terms of capital gains and dividends. Experimenting with a small amount of money now can help you avoid making a big mistake later when you have a lot more assets you're trying to manage.
