Equal-payment (annuity) keeps every monthly payment the same, which makes budgeting easy. Equal-principal pays a fixed principal plus interest on the balance — higher early payments but less total interest. On a $300,000 loan at 4% for 30 years, equal-principal saves a meaningful amount of total interest.
The interest rate dramatically changes total cost. On a 30-year loan, a 1 percentage-point increase can add tens of thousands to total interest. Always compare fixed vs variable rates and any discount conditions.
A longer term lowers the monthly payment but raises total interest. If you have spare funds, prepaying principal (after checking prepayment fees) reduces interest. Keep your debt-to-income limits in mind.