Don't waste your time getting certified. Most people in the industry don't pay attention to what the certification test is like and if you were to ask me, I would say "It must be like Microsoft where it's multiple choice versus practical and therefore, people can just memorize answers."
If you say you know linux, they'll test you on it; everything from the simple stuff to shell scripting.
If you're trying to get a job, focus on learning CentOS or SuSE. For your desktop, install Fedora since it's RPM based.
If you're doing it just to learn and play with it at home, Ubuntu or Fedora will do. LinuxMint is based on Ubuntu or Debian...just depends on which package you choose.
But in general, most(maybe all) are either based on RPM or DEB these days.
