Compare timing, cash flow, risk and opportunity cost.
A lumpsum deploys available capital at one or more chosen times. SIP spreads new contributions across periods. They are not simply two competing “products.”
If capital already exists, delaying deployment has an opportunity cost and investing immediately exposes the investor to immediate market movement. If cash is generated from salary, periodic investing may be the natural implementation method.
Use scenario analysis rather than assuming one method is always superior.