Market vs instant execution on the same broker, which one gave better results?

[Shadow Trader]

Plenty of brokers let you open two account types on the same server, one with instant execution and one with market execution, and the difference is easy to test instead of argue about. The mechanics are simple. Instant execution fills at the price on screen, or within a deviation you set, and if price has moved past it the broker either requotes or rejects. Market execution sends the order with no price condition and fills at whatever is available when it arrives.

On paper, one gives price certainty and the other gives fill certainty. In practice there are a few quirks that rarely get mentioned.

First, instant execution usually carries a deviation parameter, and a wide deviation turns it into market execution with extra steps. People leave it at a generous default and then wonder why instant fills show slippage.

Second, market execution accounts often attach the stop loss and take profit only after the fill, because the order cannot carry them at the moment of entry. On a fast spike that leaves a short window with the position open and no protection on the server side. Anyone scalping news wicks should know whether the platform does this.

Third, the spread and commission structures are often different between the two account types, which muddies any comparison. A fair test would put the same size order on both at the same moment, over a few hundred trades, and compare average fill versus quoted price as well as the rejection count.

The honest summary is that instant execution tends to suit people with a fixed entry price and a tight target, while market execution suits people who care more about being in than about the exact level. Neither is free. One charges in missed entries and the other in slippage.

Has anyone run both account types side by side on one broker, and did the fill quality gap come out where the marketing says it should?