Note that US citizens are out of reach for 95% brokers. Plus 500 isn't exception. You can still open account with themat your own risk, but if there is some legal problem, probably, there'll be nobody able to lend a helping hand to rescue you and your funds from financial trouble.
They charge no commission, but be careful – they’ll be making their money in other ways. It will be built in to the spread, and you might not get the price you want, i.e. slippage and re-quotes on exit. They have to make their £ somewhere! Once all is factored in, it’s probably more expensive but it’s hard to work out as it won’t be transparent.
Look for a DMA provider if you can. At least then you know what you’re getting.