Sofia is a dirty, badly maintained city. Quality of life is low, air is horrible and there's dirt/mud almost everywhere. It's pretty bad overall. But it's not dull, at all. Not bad for young folks because almost nobody cares about the things in their 20s.
Plovdiv is 1000x better than Sofia in any conceivable way. Older, cleaner, cultured. Built on top of an ancient Roman city. It's by far the best big place in Bulgaria
This is mostly the question of tastes. I lived in Sofia for two months, mostly in the city center. For me an international mobility is important and I don't drive, so the well connected airport and good means of transportation towards it were paramount. Thus Plovdiv and also Varna and Burgas are not an option for me. But from the business perspective it is a real tax haven. Also, there is an IT community there, and offices of large companies including Microsoft and Uber.
Plovdiv is 1000x better than Sofia in any conceivable way. Older, cleaner, cultured. Built on top of an ancient Roman city. It's by far the best big place in Bulgaria