The main difference between Instant (Instant Execution) and Market (Market Execution) accounts lies in the way your trading orders (trades) are executed.
On Instant accounts, the priority is a specific price, while on Market accounts, the focus is on speed and guaranteed trade execution.
Instant account
Instant accounts allow for high-precision entry by requiring the exact price to be available in the market. If, while attempting to execute a trade, that specific price is no longer available, the terminal will issue a "re-quote." This means it will request a new price from the server without executing the trade yet. Generally, this approach ensures the most accurate execution possible, though at the cost of extra time.
Market account
As a rule, Market accounts are used for rapid trade execution, where potential price fluctuations are disregarded. This ensures a smooth and fast trading process by allowing for price slippage. Traders are often recommended to use the "Market" account type for their everyday trading.
Which account to choose?
Choose Instant if every single pip matters to your strategy (for instance, when day trading in a calm market) and you are not willing to accept any changes to your entry price.
Choose Market if you trade on news events, practice scalping (executing numerous fast trades), use automated trading robots (EAs), or simply don't want to miss a great entry point due to popping re-quote windows. Market accounts (especially ECN/STP) are widely considered the more professional and transparent standard in trading today.