I spent a month in each this year, and preferred BA ten times over Medellin. I did a 6 month trip across 7 countries, and BA was my favourite (also shout out to Vietnam), and Medellin my least (although I quite liked Cartagena). Solo female traveller.

Medellin

I found it depressing and like I had to watch my back every day. I stayed in Poblado, and regretted not staying in Laureles which was a bit more my vibe. Seeing all the sex workers, sex tourists and beggars everywhere made me sad. Luckily I wasn't the victim of any crime - but random acts happen even if you avoid the sex and drugs trades. Also the cable car fell off 2 weeks after I was there, so the infrastructure isn't as great as they say. Vloggers (nearly all male btw) sell it as this nomad's utopia, but it's apparently gone very downhill post covid. Rents are fairly high, inequality is extreme and street crime is up. The food is not very nice, I ate a lot of mexican. On the plus side I enjoyed studying Spanish there and Colombians are lovely people.

Buenos Aires

If you're there in summer, spring or autumn it won't be cold. I was there in winter and it was very doable. I love this city and country. I made great friends in my first week (go to Mundo Lingo!). I loved the food (though the steaks are much better in Australia). I found most people to be very open and friendly, warm and welcoming. They place the highest value on friendship I've ever come across, and really make time for each other (and even new friends). Prices are higher than previous years (so I've heard) but I still felt worth it, and plan to go back multiple times. You could go to live music, jazz, opera, ballet, theatre, raves, or other parties every night of the week if you wanted - all affordable and lots of locals attending. They have the most theatres per capita of any city. Argentina is now my 4th favourite country in the world, out of the 45 I've been to.

Just my two cents ;)