Ubuntu, Novell, Mandriva, Debian and Redhat are considered the major distributions and the majority of what is out there is probably going to be based on one of these. The major distributions usually have some certification programs associated with them. Ubuntu is really pushing to get into the enterprise and it is the most popular distribution, however Redhat currently is the leader in the enterprise market. Novell is trying hard to unseat Redhat and Mandriva is extremely popular in Europe, it is a very competitive market right now.
I almost forgot to mention that Debian is very highly acclaimed for it's stability and is considered one of the best operating systems for servers today.
Something you will discover is that no matter who the distributor is, Linux is Linux. There are different methods of package management, tools and configurations but all in all anything that can be done on any distribution can be done on another. In my opinion Ubuntu and Mint would be good choices for you since you are just starting out. Both Ubuntu and Mint are Debian based and Debian package management is very easy to live with. I mention Mint not just because I am a team member but because you are wanting to try Linux as your primary operating system and you would probably be a lot more comfortable with it. Mint does have it's own custom tools but the underlying system is still basically the same as Debian and Ubuntu. You will not have to "force yourself to learn" with LinuxMint, we get a lot of people that are new to Linux and the majority have a very good experience and enjoy it.
You are wise in adding some Linux certifications to your resume. It has been predicted that by 2012 Linux will have a high market share and with the current economy combined with the advances Linux is making, I believe that this prediction could come true. I am testing the next long term support release and it is in my opinion the release that will push Linux into the mainstream market. Whatever you choose, I wish you luck and hope you enjoy Linux as much as I do.