As per Sentora DOCs concern, they do support both, so it comes down to personal choice.. The OS you previously have worked with and familiar with is the one you should give importance..

• CentOS is most widely used world wide when comes to web servers.
• Ubuntu is most widely used here in DO I see.. Hence you are suppose to get more tutorial for many software and other stuff, though off late CentOS tutorials are also getting added..

Stability wise, if configured and maintained properly, then both should be fine.

Personally I use CentOS (along with zPanel) as I felt it runs better with less RAM environment..

If you are planning on using Cpanel then you don't have a choice you need to use centos as Cpanel doesn't support anything else. Now there is other control panels out there. checkout zpanel or vesta if you want to use ubuntu

I go for Ubuntu with ISPConfig panel. The installation was nice and smooth. thanks