I read somewhere that using a discount broker such a Trade King could actually cost more money because they may not be able to get the best price on a trade. Is there any truth to that? I am currently trying to decide between Trade King and Scott Trade.
They're not negotiating trade rates for you, you set the trade rates in your order. What they might have is a slightly slower system, delivering your orders a second later than the competition would. If that's critical to you then you should look at that, otherwise look at their fees, customer support and research aids because that's where the broker value is.
Always use limit orders never market orders. Period. Do that and you will always pay what you said you would when the transaction goes through. Whichever broker you use is not going to "negotiate" for the best price on your trade if you choose a market order. Their job is to fill that order so they will always buy it for more than market and sell it for less to ensure the order goes through. It is not even a factor when choosing between TradeKing and Scottrade.
I use Trade King and my friend uses ScottTrade. Besides the transaction fee (TK is a few $$ cheaper), the only other things to consider are the tools and research (and customer service if you need it) that each site offers. I went with TK and the lower transaction fee since tools and research can be had from other sources. I basically only use it when I want to make a trade since I don't find the tools particularly useful and I never take an analyst's opinion of a stock at face value anyway since everybody always has their own agenda.