Santo Domingo was all I really knew growing up in the DR, although my maternal and paternal families were originally from Puerto Plata, Mao and Santiago. I used to visit relatives in all three places all the time. I have not been to Puerto Plata in a while, but it is definetely NOT the same town I used to love to visit growing up. I loved Mao as well, but TODAY I would lean towards Santiago. Now, I do love Santo Domingo because it is the city where everything happens (the good and the bad). From what I hear, the crime seems to be higher these days, but not as high as other Caribbean/Latin American capitals. There are many nice places to live: Piantini, Naco, Bella Vista, ect. I do recommed that you explore both Sto Dgo and Santiago. Both have great schools. The cost of living is higher now everywhere in the DR, so I am not sure if there is a difference between cities. The temperature doesn't vary much among cities, except in the mountain towns of Contanza or Jarabacoa, though I find La Vega much cooler than Sto Dgo and Santiago. Hope this helps.
Natasha