I am primarily debating between DR and Colombia as my original choice of Costa Rica now seems way too working girl touristy - sounds more like the place you go when you have someone to take or when your a man who likes to pay for sex.
I say this because Cartagena seems to be infected with the tourist town prostitution bug, Barranquilla sounds uninviting and Bogota is just supposed to suck.
I really like Costa Rica. Outside of an area with massage parlors/bars in San Jose (and one bar in Jaco) you won't find prostitution all over Costa Rica (or San Jose). I mean unless you specifically seek out the Blue Marlin Bar this just isn't an issue. The pay for play is all concentrated in one area... its not all over.
What is this prostitution kick? I hear Medellin is as popular of a sex tourist destination as Cartagena. And Cali is loaded with Prepagos.
I'm not sure what job you have that allows you to travel like this... but I say lucky you... and enjoy.
I wouldn't look past Costa Rica... if you are willing to rent by the month there are some good places to stay.
I personally crossed the DR off my list. You don't think prostitution is huge there? You might want to read up on that. It is where Rush Limbaugh goes to bust a nut.