This is a point of contention, and a somewhat emotional one at that, but there are substantial disparities between the various varieties of Arabic both in lexis and in grammar. Testament to this is the fact that all varieties of Arabic are not mutually intelligible with one another. Ethnologue classifies Egyptian Arabic, for example, as a distinct language in its own right.
The distinctions are too numerous to list. MSA for "death" (موت) is /mawt/, but in Iraqi it's /mu:t/. MSA for "I speak Arabic" is "`ana `atakallam al-3arabiya," but in Lebanese one would say, "`ana be7ki 3arabi."
If you're not interested in any specific subregion per se, do the obvious thing to do and go for MSA, which is the lingua franca and the oft-preferred mode of communication in formal settings. Command of the language is not, however, quite universal in the Arabic-speaking world, especially in some countries. If you only know MSA, you will find it difficult or impossible to understand what is being said on the streets.