To be clearer, I think those 'al' VS. 'el' are a bit pointless. In formal Arabic, you would pronounce اَلْـ as /æl/ with the sound æ (i.e. the same sound as A in pal). If you speak informally though, the vowel sound becomes shorter and and less open, therefore becoming /el/ with the sound e (i.e. the sound as E in bell).
P.S.: Here formal/informal is not related to dialects. Formal speech would be over pronounced, whereas informal speech would be more natural and fluid.