Simple interest applies only to the principal, while compound interest applies to both principal and accumulated interest. The longer the period, the more dramatic the difference — e.g., 5% annual compound rate over 30 years turns 1x into ~4.3x.
For the same nominal rate, more frequent compounding yields slightly higher effective returns. However, the difference is small — focus on comparing the nominal rate and terms of each product first.
Years to double = 72 ÷ annual rate (%). Example: at 6% annual compound rate, 72÷6=12 years to double your money.