Paper trading and replay simulation get grouped together as "practice without money". They train different things, and the difference matters if your time is limited.
Paper trading: honest pace, honest conditions
Live paper trading runs at real speed on real order flow. Its strength is that nothing is compressed: you wait, you get bored, you face the setup at the same hour of the day you would face it for real.
Its weakness is the same thing. Waiting is most of it. If your setup appears twice a week, a month of paper trading is eight repetitions.
Replay: compressed repetitions, same uncertainty
Replay gives you the same unknown future — the next bar has not printed for you either — but lets you move through it at your own pace. A weekend can hold more occurrences than a quarter of live practice.
What it cannot reproduce is the texture of a live session: the delay before you decide to look, the interruptions, the fact that you have to be at the screen when the market is open rather than when you are ready.
Where each one fits
Use replay to build the repetitions: pattern recognition, execution mechanics, sitting through an open position. Use paper trading to check that the skill survives contact with a real schedule.
Doing only paper trading is slow. Doing only replay risks a skill that works when you are fresh and focused and nowhere else.
The case for the replay half is in compressed practice time; what separates training from screen time is in deliberate practice for traders.