A Market Order is an order to buy or sell a security at the current market price, while a Limit Order is an order to buy or sell a security at a specified price or better. Market orders are executed immediately at the best available price as soon at the trading signal is sent into the exchange, while limit orders are placed on the spread at the price that the trading signal sends into the exchange. There is a risk that the limit order will not hit if the price action on that asset is highly volatile. There is one factor that you do need to take into consideration with a Market Order, and that's Slippage. Slippage is the difference between the expected price of a trade and the price at which the trade is actually executed.
It occurs when there is a delay between the time a trade is ordered and the time it is filled, and during that delay, the market price of the asset being traded moves. Slippage can result in either a higher or lower price being paid or received for the asset, and can occur with any type of order, but it is most common with market orders. Slippage can be a significant factor in high-speed, automated trading strategies.