|farsi versus dari - They're essentially two dialects of the same language, but I believe that, as a learner, you're probably better off seeking to master one rather than mix the two.
Farisi is spoken in Iran, while Dari is spoken in Afghanistan. The vocabulary differs to a fair degree, as does the phonology and even some of the grammar. Dari speakers see Farsi as being "effete", while Farsi speakers see Dari as crude.
Mutual comprehensibility is no great problem for native speakers of either, but as a foreign learner of one or the other, you're probably best to try to master one first before branching out.