Hey...sorry for the late reply. I missed this.
I actually haven't been to chiang mai (lived in Phuket), but still have an idea of how it'll likely be.
Here's my answer
1) Community of DN entrepreneurs - that hands down goes to Chiang Mai. Chiang Mai is arguably the biggest, or at least most populated DN spot in the world.
2) Quality of Life - I think prices in Chiang Mai have gone up recently, while Medellin prices seem to keep falling. Are you using the dollar? If so there's never been a better time to get to Medellin, price wise. Your money will go really far.
That said, it's still Thailand and Chiang Mai we're talking about. So I would still say your money in terms of rent/food will probably go a bit further there, but Medellin isn't far off
3) Cool experiences - I guess that depends. You're a guy...and if women are important to you, Medellin has some of the most beautiful women in the world, hands down. In terms of other experiences, I know there are a lot of cool things to do in Chiang Mai, but Medellin has some fun stuff too.
So factor that in. I think you can't go wrong with either personally but...as a first time DN, if you want a lot of cafes / coworking spots to work out of, interact and easily meet lots of DN's, and save more $, I'd say Chiang Mai would be good as a first stop. Then once you're up and running you can always check out Medellin after for a better perspective.