sorry i didnt make it clear. tdameritrade doesn't update margin on a real time basis. you only have a certain amount of buying power per day, and if you use it up, you cannot make any more daytrades. with IB, theoretically, you can make infinite number of daytrades as long as you are not over the 4:1 margin anytime during the day. Say Acct value is 100K. Td, you have 400K intraday buying power. You buy stock 100K, sell stock 100K. Repeat 4X. You cannot make anymore daytrades. IB, you can.
At Ameritrade, if you have $100k cash at the beginning of the day, you will have DTBP available $100k x 4=$400k. You can buy/sell $400k (DTBP) worth of stocks multiple times, your DTBP gets updated real-time. The advantage is unlike IB, you can fully use 4x buying power without getting automatically liquidated for the slight fluctuation in stock price.
$5.00 trades for stock and options with a TD Ameritrade IZONE Account. This is only for self-directed investors/traders who do not need any sort of customer service rep TELEPHONE assistance whatsoever. All Client Service inquiries are via e-mail. http://www.izone.com/savings.html
i know its not good for daytraders, but for my personal account i use wells fargo, you get 100 free trades if you have 25k in assets with them