Usually you have to have a really terrible 401K - front loads, very high ERs, that sort of thing, for taxable to be better than the 401K. For example, I have a solo401K - no match that doesn't come from my own pocket, and yet I still do it. Why? Because it saves a boatload in taxes, and the funds I've chosen aren't terrible.
But do your own math / post more details if you want a better answer.