Both use fake money. Only one lets you replay a crash before lunch.
Paper trading means placing orders against real, live market prices, in an account that quietly never sends them anywhere. The prices are real. The money is pretend. Most brokers offer it, usually as the same screens as the real account with a different label on top.
A market simulation builds the market itself. Its own companies, its own prices, its own economy, none of it plugged into a real exchange. Margin Call is the second kind.
Learning one platform. If you're going to trade through a particular broker, their paper account teaches you their order tickets, their shortcuts and their quirks. All of that is real knowledge, and no simulator can hand it to you.
It also follows the actual market, so anything you test meets the conditions that really exist this week.
The weaknesses are built in. It runs at the speed of the calendar, so testing anything takes as long as it takes. It gives you one history, the one that happened, and you can't ask it for a bear market. And because your orders never reach a real order book, paper fills come out better than real ones would. You get the price on screen without the slippage your size would really cause, and that flatters every result.
Squeezing time. One run of Margin Call covers about five years and fits into an evening. You can sit through a full rate cycle, a recession and a recovery, then start again with different conditions.
Repeating with variations. The same seed rebuilds exactly the same market, and a different seed builds a genuinely different one. So you can check whether an idea survives conditions other than the ones that happened to come up.
Charging you properly for size. The simulation owns its order book, so it can make big orders cost more. In Margin Call the depth you see is the depth you trade through, and a large order pays slippage by eating price levels. You end up much closer to real trading than a paper fill at the price on screen.
Showing you causes. Rates feeding through debt into profits. Holdings falling together in a crash. Option prices collapsing after results. You can watch all of it happen, because the simulation holds the cause as well as the effect.
The psychology. Both are free of consequences, and consequences are the part experienced traders call the hard bit. A bet that feels fine in a browser tab feels very different with real money behind it. No amount of practice in either one gets you ready for that.
Treat both as ways to learn how things work. What an instrument is. What a mechanism does. Why something behaves the way it does. Neither one proves a strategy works.
If you already trade and want to rehearse on a particular platform, paper-trade there.
Want to understand why markets do what they do? Want to see lots of different conditions, instead of just this month's? Use a simulation.
Margin Call is free, needs no account, and runs in a browser. Nothing in it is financial advice, and nothing in it connects to a real market.