If you want to live in an outdoor sauna ,close, but still too far from the beach, then Barranquilla is absolutely for you
Just remember, Shakira and Carlos Vives were only pulling your leg about cycling there, you'll be the colour of a fire engine before you've gone a block.
My vote is for Cartagena, despite the annoying vendors, despite all the tourism. It's Latin America's equivalent of Venice, for that you can cut it some slack. And it has beaches everywhere.
Too hot and humid in Barranquilla. Cartagena is by far the most beautiful city in Colombia but just too small and touristy. After a while I think it would get old. The beaches in town are dirty and suck (although they have nice beaches a short boat ride away but let's be honest it would be a pain to have to go that route vs. just walking). No doubt the old town is beautiful but I much prefer bigger cities.
I didn't think the food was anything special in Cartagena. The best places are mom and pop and not too expensive (try Narcobollo which is my favorite spot in Cartagena... where the locals go). Brazil and Colombia are the two best countries to find a beautiful, educated and intelligent girl.
Many of the hottest girls in Cartagena are prostitutes which I doubt you'd want to deal with if you want a meaningful relationship. And many of the others are dark skinned which isn't my personal taste. In Bogota, Cali and Medellin it's not difficult to find a meaningful, intelligent and beautiful non-puta. In fact, as a gringo you're a commodity there so you will get a lot of attention. Before getting married, I'd travel around much of South America so I know it well. Not all, so I don't want to generalize. But after living and working in Argentina for the past 20+ years I know it well.
I had lots and lots of employees and they were almost all single. Even today most of them are single in their 40's. The ones that WERE married are now divorced and likely won't get remarried. And the guys are no better. Many don't want to ever get married and would rather hang out with their bro's vs. hanging out with a girl. Weird culturally that society that way.
I mean on one hand it's cool if you're a guy and you meet a good group of guys because you will be a brotherhood and these guys would kill for you. They would do anything for you and I made some lifelong friends in Buenos Aires that I'm closer to than my real family. But sometimes I'd yell at them because they would rather hang out with the bros even when a beautiful girl would want to go out.
I thought the most beautiful girls were in Cali. 2nd was Medellin. The girls are so beautiful in Colombia I'd think your biggest issue/problem would be trying to stay faithful to your girlfriend. Because as a gringo, girls will always be attracted to you (at least that was my case when I visited). I'm not just talking about just prostitutes (who would be constantly giving me their number on paper and definitely will be attracted to you). But just walking around in the mall I'd get people that would chat me up while paying for something. Or in restaurants, or stores, or pretty much anywhere. I don't think I could remain faithful in Colombia. In Argentina you won't have that issue. Most of the females are a royal pain in the ass.
Your priorities might be difficult. If you're looking for an amazing wife, then Colombia is a good option. If you do it right you can find a really intelligent girl. Colombians are the best wives and best mothers. I should know. I've been married to one for the past 17 years.