I've not lived in either Medellin or Bogota but have spent about 7-8 months travelling in Colombia including a good bit of time (over a month) in Medellin.
For me Bogota is kind of like Lima or DF - tons to do, a vibrant metropolis, anything you want available, but feels like the big city it is, and air quality can be pretty poor. Traffic in Bogota is horrendous - there is no good public transit and most roads are 1-2 lanes in each direction, for something like 10 million people. I believe it regularly ranks among the worst traffic in the world. It's a fairly gray and cold/rainy city if that matters to you.
Medellin is ~3 million people I believe and has pretty ok public transit in that it has a really excellent light rail that runs the length of the valley (one main track basically and the city is kind of elongated down the valley) and then pretty fair busses. Traffic is very manageable in my experience, certainly no worse than BsAs. The hip parts of town (notably Poblado and Laureles) can feel a bit like Palermo and Belgrano at times - hip cafes, lots of craft beer, lots of good food. It's not Argentina obviously, still has Colombian energy, and the streets are quiet much earlier than BsAs, not surprisingly. There's a lot of nice architecture, and the city feels pretty young and worldly, though not as worldly as BsAs. The climate is great in my opinion - pretty warm, somewhat humid, rains frequently. Medellin and BsAs are my favorite Latin cities, hands down.
Popoyan is lovely, a colonial town called the "white city" that is really scenic. It is quieter, a bit more reserved, buy young as it's a university town, really lovely but not really worldly. Cali is thriving and bustling, and hot and humid, and seems quite appealing in many ways but I don't know it well. Cartagena is kind of a big, sprawling, ugly morass except for the old city which is really gorgeous, up there with Granada (NI) and Trinidad (CU) for me as my favorite colonial towns, really is worth a visit at some point. The other Colombian cities on the Caribbean - Baranquilla and Santa Marta, are both quite nice. Baranquilla is really laid back and a nice balance between modern and sleepy. Santa Marta is a bit more of a backpacker's and partier's hub but is beautiful and is close to Minca and the mountains as well as PN Tayrona as well as lots of other nice attractions. There are some other nice smaller cities in Colombia I like as well - Valledupar and Villa de Leyva are pretty well known among travellers. The only towns in Colombia that I've been to that I didn't really enjoy are Bogota (traffic and air), and Pasto (I only have passed through twice and it just struck me as kind of a grubby crossroads but I know others have higher opinions so don't read too much into my comment).
