I've had/have accounts with both Ameritrade and E*Trade. As others have already pointed out, trades are faster with Ameritrade, and in general I have found Ameritrade easier to work with.

Since this is the MilesBuzz forum, let me add something about miles. When I initially opened my E*Trade account, I clearly qualified for the signup bonus. When no miles appeared even after having waited beyond the prescribed waiting period, I contacted E*Trade. They waffled on the miles, and it took several rounds of back and forth emails to finally get said miles.