The core difference is simple: when you pay tax. The right choice depends on whether your tax rate is higher now or expected to be higher in retirement.
Traditional
Contributions are pre-tax, lowering your taxable income today. You pay ordinary income tax when you withdraw in retirement. Good if you expect a lower tax bracket later.
Roth
Contributions are made with after-tax money, so there's no deduction today — but qualified withdrawals in retirement are completely tax-free, growth included. Good if you expect the same or higher tax bracket later, or you're early in your career.
Why many people split
Holding some of each gives you tax flexibility in retirement, letting you manage which account you draw from to control your tax bill.
Either way, contribute
The account type matters less than consistently contributing and letting compounding work. Project either with the calculator.