There are lots of good services, and they all provide similar features.
However, here's a thought. Most investment managers recommend a strategy that does not move investments around very frequently. In fact, a semi-annual, or even annual review is generally acceptable.
If you are talking about day-trading, then you are basically talking about gambling. The beta is huge, but cuts both ways. As a rule, day-traders make a lot of money in a bull market, only to give it all back and then some in a bear.
I highly recommend educating yourself on the market. A WSJ subscription will net bigger returns than a day trader account, in the long run.
Invest wisely for the long term, manage risk through diversity and time, and you'll make money.