Fully Taxable: This is the worst kind -- compared to the other two -- since your money is taxed when you save it and is taxed once again when you withdraw. Example, money you place in savings accounts or regular mutual funds (outside IRAs).
You aren't really taxed when you withdraw, you are just taxed on your earnings. So, if you have $100k in a savings account at 1%, and you spend half of it in a year ($50k) you are taxed on the 1% you earned, not the $50k you spent.
That being said, we max out our 401(k) for the tax deduction because we are in a higher tax bracket now than we expect to be later. We also contribute to a Roth because the first $20k of income isn't taxed anyway.
So, for example, let's say we need $30k to live in retirement. The first $20k or so isn't taxable (standard deduction/exemptions). So, we'll pull $20k from our traditional 401(k), and the other $10k from Roth (also not taxable). In effect, paying no taxes. That's all really quite theoretical, as many factors can change, but that's the general idea of why you want money in multiple buckets.
We also save in taxable accounts for flexibility. For example, while there are ways to buy real estate within retirement accounts, it is more complex than buying it with post-tax money.
I can't really help you with percentages, since we max out the tax-deferred/exempt options. It is really going to depend on your income, goals, timeline, investment preferences, and guess about future tax rates.
