Almost unanimous for Medellin over bsa is good enough for me. Now i need to find an apartment any advice?
Honestly I would not use the consensus of BAExpats to make a decision. In general there is a lot of complaining on this site. Complainers are in general louder than happy people, so you're getting a bit of a skewed take.
I've been to both and I'd say the following
Food: BA has better restaurants, this is mainly because it is a bigger city. Yes there are a lot of crappy resto-bars in BA, but there are some really great restaurants. Medellin has some restaurants, but is not really on the same level as BA. Medellin does have more fresh fruit and veggies. BA has better meat.
Nightlife: BA has better nightlife, hands down. Same thing here - its a lot bigger, more international city so there are a lot more options.
Niceness: Colombians I think are nicer/less arrogant that porteños. I think you'll get that vibe though when comparing any large city with a smaller one (e.g. New York vs. Indianapolis, Paris vs. Lille).
Cost: If you have dollars its about the same. If you'll be earning a local salary, Medellin is cheaper.
Security: Both are relatively safe. I'd say toss-up there.
Spanish: echo the board here. Colombian spanish is more neutral and easier to understand. I think there will be more language schools here in BA, but I'm sure you'll be able to find one in Medellin.
Expats: If you want to have friends, then either place you'll likely be relying on English-speaking expats. I think BA is better for that aspect. Personally I love my expat friends, it is one of the best parts of living in BA. I'm pretty sure there is a much larger expat community here in BA. Medellin is growing as a destination, but it is still saddled with people thinking it is a drug lord haven, so not as many people go there. From my experience there are a lot of sex tourists in Medellin.
Cost of Flight: A flight to Med from the US is roughly half of flying to Argentina, sometimes even cheaper.
Culture: BA has more cultural activities (concerts, art galleries, theatre, etc).
Outdoors:The outskirts of Medellin are prettier than BA. That being said, Argentina has more awesome places to go as a country (patagonia, Iguazu, Salta, etc). So that depends on if you want to travel.