When choosing a mining company the first thing you should ask is what do I know about the owners and will I be able to contact anyone if they bail.
The more personal details they disclose the better, but these of course have to be verified. So if a company doesn't give out their address and name of the owner, you shouldn't even consider investing.
It doesn't matter if the site is paying out, most of these sites aren't paying for more than 2 or 3 months, so you can't really call them stable.
This is completely irrelevant when you have essentially waived all of your rights in the terms of service. I suggest reading it - knowing the operators does not help if you have zero legal recourse if they shut down everything tomorrow.
-Sahra