Two different questions
If you already hold a large amount and are deciding how to deploy it, that is one question. If you are investing out of monthly income, there is no decision to make — a SIP is the only option available to you.
Most of the argument on this subject conflates the two.
If you have a lump sum
Markets rise more often than they fall over long periods, so investing everything at once has, on average, ended up ahead of staggering it in. That is a statement about averages, not about your particular month.
Against that: deploying everything a week before a sharp fall is an experience many investors do not recover from behaviourally, even when the portfolio does. Staggering the entry over six to twelve months through a Systematic Transfer Plan gives up a little expected return in exchange for a much lower chance of a decision you cannot live with.
For most people that is a trade worth making — not because the maths favours it, but because the maths assumes you stay invested, and staggering makes that more likely.
If you are investing from income
Then you are running a SIP by definition, and the only real questions are how much, into what, and whether you keep going.
The value of a SIP is not primarily rupee-cost averaging, useful though that is. It is that the decision is made once and then executed automatically, removing the monthly temptation to wait for a better level.
What actually decides the outcome
- Whether you continue through falls. The investors who do best are rarely the ones who picked the best scheme.
- Whether you step it up. A ten per cent annual increase compounds into a materially different corpus.
- Whether the horizon matches the asset. No entry method rescues equity money needed in two years.
The entry method is a second-order question. It gets most of the attention because it is the one that feels like a decision.