I know both well - I looked after them both for a ski holiday company years ago.
Borovets = poorer infrastructure - older lifts, older hotels, cheaper, fewer russians, more prostitutes, but sort of fun in a way. not a bad place, but certainly not that great for skiing to be honest.
Bankso = better skiing. newer hotels, more expensive. better lift system - all lifts are modern four or six packs. can be a bit busy queuing at the bottom of the mountain in the morning at the gondola (only way to get up the hill). realistically, neither are amazing and can be skied in a day. BUT, if theres a dump of snow it'll be great fun.
Bulgarian food generally is served warm rather than hot - most restaurants and hotels will accommodate british tastes if you ask for food to be served hot. i found the food to be very good on the whole, lots of stews, dumplings etc. heavy winter food which sounds fine to me when I'm in the mountains.
neither is that cheap any more unfortunately, but, it's cheap and cheerful and does what it says on the tin. friendly locals too and some good nightlife in Bansko (but you've got to go looking, which is part of the fun!)
Don't go expecting 3 valleys of dolomiti superski and you'll no doubt bump into some sound people and have a bloody good holiday.
One other thing to note is that ski schools are actually VERY good, far better than anything I've experienced from the likes of ESF.
Hope that helps!