Well, first, strictly speaking, mandarin Chineses is not a "dialect". It's just like RP (Received pronunciation) in English or sort of "standard English". It is built on our north dialect (Chinese) and works as our offical language ("standard Chinese"). It is not only popular but an requirement for every Chinese people to learn, so as a beginner, you'd better choose the mandarin Chinese，in terms of accent (pronunciation, tone, grammar etc.).
Then, when it comes to the "version " of the Chinese characters. There is no doubt that the traditional characters are easier to understand and memorize, because, compared with the simplified Chinese characters used in the mainland, they represent more rules of their creation. Thus, you may choose any "version" you want. However, if you didn't learn any simplified characters at all, you would be in trouble with communication when you travel in the mainland, China.