Personally I think DAX is the one of the two which is syntactically more coherent with its subject. That is, unlike MDX, Microsoft haven't tried to make it look like SQL (the "SELECT something FROM something WHERE something" paradigm. Although I was once told that originally the keywords were different (CONTEXT instead of WHERE, but I suppose you'd have to ask Mosha Pasumansky!))

What I'm saying is learning DAX gives you a better understanding of the concepts of SSAS Tabular, whereas MDX can lead to confusion when used in the query form, especially if you come from a SQL background. If you find yourself in the latter camp then Frank A. Bannin wrote an unsurpassed article on this very site on the subject: http://www.sqlservercentral.com/articles/MDX/91228/

As others have said though, good luck! It is tricky but it's worth it and (I think) a lot of fun.