Limit orders are superior to market orders. At many brokerage firms, including Scottrade, limit orders are the same price as market orders, so there’s really no excuse not to use them.

Limit orders allow you to set the worst price at which you’ll by or sell a stock or option contract. I say the worst price, because you might get an even better price than the one you ask for. If you place a market order then you’ll buy at the ask price and sell at the bid price. This will generally make your purchases more expensive. For that expense, you do tend to get faster execution, since you’ll buy or sell at the current price without having to wait for someone to match your offer. The speed is nice, but I’d much rather get a good deal slowly than a bad deal quickly and have more control over the price I pay.