Medellin and sosua are 2 different worlds. The weather is different and so are the people. There is no beach in medellin. Not easy to talk to women and certainly they are not easily charmed. There are good areas and bad areas in the city. bad areas could be very dangerous and the good areas could be expensive. Most foreigners go to any city and end up in a whore house and then come back to tell tales of their conquests. If this is what you have in mind, then any city in colombia will fulfill your requirements.
As compared to sosua, in medellin, no one will even notice you, much less talk to you. There is plenty to do and see there, but there is no beach in sight and no vacation type atmosphere. Its a big city. I find medellin to be a bit more expensive, even more so than bogota but good deal can be found if you know where to look and if you speak good spanish. Of course speaking spanish is not enough, you have to be able to blend in well with south american culture as well. Know the norms in advance and learn to speak like them. Dominican spanish is not considered very elegant language to them. If you don't want people making faces of dried prune, watch your tongue. Colombians speak better Spanish than even Spaniards (in Spain).
I have also come to realize that gringos (especially the back packers) have screwed it all up for the new visiting foreigners there. When you tell folks you are a foreigner, they instantly think we are here to meet a paisa (girl native of medellin or the department of antioquia). There are a ton of marriage agencies and call girl services and even dating services that cater to gringos. The back-packers have given out a bad impression of being cheap and going the
dutch way when they are out on a date with a girl. No girl is expected to pay when she is accompanied by a guy. No decent guy in latin america (even dominican republic) lets the girl pay on a date or dinner. So if you think you can go dutch with a girl, forget it. Foreigners already have a bad rep there. The colombian girl will not talk to any guy on the street or in a bar. In order for you to score there with non-hookers, you have to know them in advance, before you even go there. try
Tagged and meet them there first.
Watch out for fake currency handed to you by cab driver in the dark. I was passed one, but it was only a 10K bill ($5 US dollars) so no big deal but some people can really give you big fake bills. Stay away from barrio type girls, you can easily get drugged and lose everything to the thieves. I would stay clear of bad areas. just be careful and don't act innocent. If you are the innocent type, stay away from colombia. Its for the big dogs. If you take your precautions and play the game right, you will be addicted to colombia in no time.
Good luck.
AZB