Probably this question has been asked million of times and each time a new foreign speaker wish to learn Arabic will ask this question to make a choice
Answer has ranged between Arabic Language passionate and advocates who see Modern Standard Arabic the only route to learn Arabic and on the other hand other Arabic Native speaker who believe they are offering much easier route by learning Arabic Dialect.
I like the short answer I read in this web article as it comes a learner rather than a teacher. The learner is foreign speaker who practiced herself learning Arabic Language
She believes that there is NO one size that fits all learning needs.
As the need to practice grows and diverse, the need to use and derive more Arabic vocabulary will arise more. The need to read and write and follow digital and printed media will arise and so on. Here come the advantages of structure, logic and resourcing different words and vocabulary to new words using the root for these words, The Gerund. This structure and logic in rules are only easily learned in Modern Standard Arabic.
Then again, the more learner advances in their fluency , the more social they become which again raise need for more ways to communicate with other people, such as exchanging jokes which are much funnier if you say them in Dialect! . This is an opinion again and no one rule apply to all people.
Stories of people who faced challenges when they learned one form of Arabic Language -Standard or Dialect- and not the other are many and we are curious to hear your story, so please don’t be shy to share that 🙂