Where do you live? If its Australia/ Asia I don't see any reason to trade US markets as a daytrader. There are more than enough markets with opportunity in our time frame rather than having to start "work" at 11:30 PM!!
With that said that doesn't necessarily make the ASX the natural market to trade. IMO Asian Index futs, FX, and Euro markets are where its at. At a stretch maybe Aussie SPI if you enjoy watching paint dry and if your really really Xenophile then ASX20
Now of course from this line,
You will know what to trade. As no one should trade without having already tested their approach/system and achieved and know why they archived profitability. If you're about to go live you already have the results to answer your question.
On the broker Qs. Most of the frequent traders here use Interactive brokers. The Aussie based brokers have horrendous front ends and amazingly crap brokerage rates.