Start with the exact problem and affected quantity: glass, sash, hardware, weatherstripping, frame, water path, rough opening, operation, security or a broader comfort goal. Consider extent, repeat pattern, compatible parts, surrounding material, warranty, historic constraints and whether a durable limited repair is available. Age alone and one failed unit do not decide the answer.
Compare a repair scope with insert and full-frame replacement only where those options are relevant. Each should state what stays, what is removed, how water and air boundaries are handled, what finish work is included and how concealed conditions change the job. Repair remains a valid editorial outcome; a separate commercial replacement path applies only when at least two windows are genuinely being replaced or installed.