What are the differences between market and instant execution?

When Trading in fast-moving markets, the prices at which you can open positions often change by the time you place your order. The key difference between market and instant execution in such cases is whether your order gets filled, or not.

With market execution, your orders are always placed at the best available price. While, with instant execution, your orders may be rejected, leaving you with a choice of whether to place them at the new, requoted price, or not at all.

Market Execution

Faster (orders executed in milliseconds)
Guaranteed to execute orders with no requotes (price comes directly from the market)
More prone to slippage during high volatility (may result in better or worse prices)
Best execution for fast-moving markets
Lower spreads

Instant Execution

Takes longer (orders only processed after confirming requoted prices)
Often results in requotes
No slippage (you decide whether to accept requoted prices)
Results in missed opportunities in fast-moving markets
Often involves fixed spreads (wider than the real market spreads)