Not so sure, but I do know that if you have a smallish sized capital to trade with, then cmc is your best bet. Since you don't pay a commssion cost eg. minimum of £x a trade like you do with GNI - though of course do remember you are paying for a slightly larger spread - that's how they make their money. So therefore you can trade in far smaller amounts, diversify your portfolio and obviously save on commision costs for much smaller and more frequent trades.
Apart from that I think GNI account requirements are higher than CMC, you may need more capital and more experience. Although I was turned down by CMC because I didn't have a mimimum of £20,000 in cash or stocks needed to open an account. That's what you get for being honest.