If it was me i would take medellin. It is cheaper, people are friendlier, they speak better spanish - or at least i understond alot more than in BSAS. Crime is less, and police actually do their job. The city is cleaner, the subway works, and is modern and clean, accomodation is cheaper and of better quality, and there is alot of stuff to do close by, but outside of the city - as in hiking, camping etc. The food is both cheaper and of better quality - especially if you like tropical friuit, but even meat etc is cheaper or same price and of better quality. Plus you get a way better variety of food compared to BSAS, whether restaurant or supermarket.
Weather is better in medellin too, warm during winter, and not hot and humid during summer like BSAS - they say its is the city of the never ending spring in medellin.
I also never got ripped off because i was a foriegner in medellin, unlike BSAS, and its very easy to buy or sell foriegn currency.
The only downside for me was no direct flights to countries outside of south america, i had to go to bogota.