Both use fake money. Only one lets you replay a crisis before lunch.
Paper trading means placing orders against real, live market prices in an account that does not send them anywhere. The prices are genuine; the money is not. Most brokers offer it, and it is usually the same interface as the real account with a different label.
A market simulation generates the market itself — its own companies, its own prices, its own economy. Nothing is connected to a real exchange. Margin Call is the second kind.
Learning a specific platform. If you are going to trade through a particular broker, practising in their paper account teaches you their order tickets, their hotkeys and their quirks, which is real and useful knowledge that no simulator provides.
It also tracks the actual market, so a strategy you are testing meets the conditions that genuinely exist right now.
Its weaknesses are structural. It runs at the speed of the calendar, so testing anything takes as long as it takes. It gives you exactly one history — the one that happened — and you cannot get a bear market on request. And because your orders never reach the book, paper fills are typically better than real ones would be: you get the quoted price without the slippage your size would actually cause, which flatters every result.
Compressing time. A run in Margin Call covers about five years of market time and can be played in an evening, which means you can sit through a complete rate cycle, a recession and a recovery, then start again with different conditions.
Repetition with variation. The same seed reproduces the same market exactly, and a different seed produces a genuinely different one. You can test whether an approach survives conditions other than the ones that happened to occur.
Modelling cost honestly. Because the simulation owns its own order book, it can charge you for size. In Margin Call the depth displayed is the depth you trade through, so a large order pays slippage by consuming price levels — which is closer to the real experience of trading size than a paper fill at the quoted price.
And it can teach mechanisms directly. Rates reaching earnings through debt service, correlations rising in a crisis, volatility crushing after earnings: these are all visible as causes in a simulation, because the simulation contains the cause.
The psychology. Both are free of consequence, and consequence is the part experienced traders describe as the difficult one. A position size that feels comfortable in a browser tab does not feel the same when the money is real, and no amount of practice in either format prepares you for that.
Treat both as ways to learn structure — what an instrument is, what a mechanism does, why a thing behaves as it does — rather than as evidence that a strategy works.
If you already trade and want to rehearse on a specific platform, paper-trade there. If you want to understand why markets do what they do, and to see many different conditions rather than the one currently outside the window, use a simulation.
Margin Call is free, needs no account, and runs in the browser. Nothing in it is financial advice and nothing in it is connected to a real market.
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