A market order says "fill me right now at whatever the best available price is." A limit order says "fill me only at this price or better." That single difference - speed versus price control - is the whole decision.

The trade-off

Market orders prioritize certainty of execution. You will get filled, but not always at the price you saw - especially in fast or thin markets, where the price can move between the click and the fill. Limit orders prioritize price. You will never pay worse than your limit, but you might not get filled at all if the market runs away from you.

Slippage: the hidden tax

The gap between the price you expected and the price you got is called slippage. On liquid instruments like SPY with tight bid-ask spreads, slippage on a market order is usually small. On thin options far from the money, or during a news spike, it can be severe. Slippage compounds: a few cents per contract, dozens of trades a week, adds up to real money - and it is invisible in your P&L because you never see the fill you did not get.

Speed or price - you rarely get both. Choosing which you need is half of good execution.

Why execution is a system problem

Deciding market-vs-limit in the moment, under pressure, is exactly where humans slip. Automated systems can route the order type dynamically based on how the tape is moving - using a limit when the book is calm and accepting a market fill when speed matters more than a penny. That is execution as a repeatable process, not a gut call - the same discipline behind waiting out the opening range instead of chasing it.