I encourage you to reread the AC's post above this, because I don't think you're quite seeing his point. If the government's taking their cut, it may not really matter if they do it up front or later, it's likely to work out close to the same either way. Paying 20% at the beginning and then letting your money grow, or letting a bigger pile of money grow and then paying 20% at the end, you personally have the same amount. Yeah, you're correct that the 20% at the end is bigger (i.e., more total tax paid to the government), but the amount YOU get to spend is still the same.
If you don't believe me, run some numbers to test it. A quick spreadsheet will demonstrate that it works out even.
As the AC said, the reason to game the system is if you have reason to believe your future tax rate will be higher or lower than your current rate, then obviously pick whichever investment is favored. There's also issues like if you go all ROTH and then have no income at all upon retirement, you may be throwing away your annual personal tax deduction, whereas if you've got some small amount of theoretically taxable income, you may actually get it all tax free, or at a pretty low rate after deductions. Other reasons to choose traditional: a small tax deduction when you're young and struggling to get established may be more important than tax-free income later in life, when you're well established and (if you invested well) money isn't tight.
I think for most people either option is likely to work out pretty close to the same. When in doubt hedge your bets and do half of each, if possible.
Stray note: it's not capital gains inside a retirement account, you're taxed on your withdrawals as if they're income.