Seconding previous - if conference is in Downtown/Brickell area, FLL is cheaper and has Metrorail.

If conference is elsewhere in *greater Miami area* then you should look at transportation options + distance. It's spread out, always under construction, and has traffic at all hours. If convenience is a priority, spend wisely.

FWIW, my Ex is a Miami native and once worked at MIA. We flew FLL every time.