Rather than ask, "why are these guys so cheap?", you should be asking why the big names are so expensive. :) Marketing spend plays a big role there. Getting babies to shill for your company during the super bowl requires a heck of a lot of commissions.
Due to the difficulties involved in setting up a brokerage, it's unlikely that you'll see a scam. A brokerage might go bankrupt for random reasons, but that's what investor insurance is for. "Safeness" is mostly the likelihood that you'll be able to get access to your funds on deposit with the broker. Investment funds are insured by SIPC for up to $500,000, with a lower limit on cash. The specific limits vary by broker, with some offering greater protection paid for on their own dime. Check with the broker -- it's usually on their web pages under "Security". Funds in "cash" might be swept into an interest-earning investment vehicle for which insurance is different, and that depends on the broker, too.
A few Forex brokers went bankrupt last year, although that's a new market with fewer regulatory protections for traders. I heard that one bankruptcy in the space resulted in a 7% loss for traders with accounts there, and that there was a Ponzi-ish scam company as well. Luckily, the more stringent regulation of stock brokerages makes that space much safer for investors.
If you want to assess the reliability of an online broker, I suggest the following:
- Are they a Forex broker? Definitely give these guys closer scrutiny.
- Check their registration, at SIPC, NFA, or FINRA.
- Look for ratings from trusted publications such as Barrons, which gives these two 4+ stars
- Check out what other investors are saying, such as at a site like brokerage-review.com.
It's tempting to look at when the brokerage was founded. Fly-by-night scams, by definition, won't be around very long -- and usually that means under a few months. Any company with a significant online interface will have to have been around long enough to develop that client interface, their backend databases, and the interface with the markets and their clearing house. The two brokerages you mentioned have been around for 7+ years, so that lends strength to the supposition of a strong business model. That said, there could well be a new company that offers services or prices that fit your investment need, and in that case definitely look into their registrations and third-party reviews.
Finally, note that the smaller, independent brokerages will probably have stiffer margin rules. If you're playing a complex, novel, and/or high-risk strategy that can't handle the volatility of a market crash, even a short excursion such as the 2010 flash crash, stiff margin rules might have consequences that a novice investor would rather pretend didn't exist.