... I can either go to Sofia or Cluj-Napoca for 2 days in October.

Has anyone been to either places? Did you like them?

My first thought is to go to Sofia because it will probably be warmer.

This is just for a quick weekend trip to fully take advantage of the fact that I have to drop someone off at the airport in Rome so I might as well find a cheap flight and go somewhere for the weekend.

As I don't think too much re-building has been done in Romania, I would definitely go to Sofia! I visited Cluj many, many years ago when my school (yup in the stone age!) had to switch a school trip around after there had been a big earthquake in Romania. So part of visit included, instead of a Black sea resort and Dracula's castle, a dismal, grey industrial town with very little to recommend it! I seem to remember a few pretty churches but the rest was the pits! Go to Bulgaria!!