Got tired of IB's customer service and high commissions for large shares and am currently in the process of switching to Scottrade. However I just read that Scottrade's slippage is horrible. How much worse is it than that of IB? 50% worse? 100% worse?
Look for data on the broker's web site about how well they fill orders. In particular, the percentage of trades that are executed at a price better than the limit price. You might find that higher commissions pay for themselves.
When I place a market order, 99% of the time the execution is instantaneous and at the quote on the screen. Over the last four years you could count on the fingers of one hand the number of times I went, "WTF???" because of a bad execution. And those were from GTC stop orders where prices were moving against me very fast. Scottrade gets slammed a lot on ET from people who don't like the fact that they sell their order flow. Sellling order flow is a non-issue most of the time. In fact, if you use limit orders, it's irrelevant. I haven't done any day trading on Scottrade since the late 90's/early 00's, so if you are very actively trading throughout the day, my experience isn't very helpful. Back when I did daytrade, I was using limits most of the time so it didn't matter if I was using a retail broker or if I was standing on the floor of the exchange, I either got the price I wanted or there was no trade.
I used IB several years ago and I have never used Scottrade. So I cannot give a comparsion of the two. But I trade intraday and if you are a day trader, it would appear one would be better with a firm where you can obtain the adding liquidity rebate and eliminate the spread. Also, I like direct access rather than my orders being directed to a third party and my broker receiving the rebate in payment for order flow in addition to paying a higher commission. I use mostly limit orders but market orders should be filled quicker with direct access rather third party transactions even in fast markets. To each his own.