I just opened an IB account for short-term trading. For longer holds, though, I'd like to take advantage of the flat-rates that some other brokers offer. I know these are generally lousy brokers, but doesn't it make sense to use them for 10,000-share limit orders that will be held for one week to a month? I know the order execution is far and away better at IB, but if I want to trade 20,000 shares of S, then my round trip is going to cost about $100 through IB. Whereas at some place like TradeKing, it would be only $10. So in these types of trades, who would you recommend using? Or does it really not matter?