Been to both. Boro better for nightlife. Skiing harder in boro and in my opinion not so well organized. This is not just a one off view as I have been 5 times and enjoyed them both for different reasons but only going out once this year and have chosen Bansko as I think the slopes are better maintained. Iam sure you will enjoy boro. Go see Christo in the white magic as he makes it special.
The Ela is undergoing a complete refurb at the moment, so will be all nice and new for you. Personally im a Boro fan, i think the slopes are a lot more varied, and theres a vast amount of new signage going in now as well