I currently live in Boerum Hill and have lived in Ft Green. You can't go wrong with either choice IMO. Great hoods, safe, loads of chicks. Pk Slope is cool because of the access to the park. I can get there in a few subway stops. I can walk to Ft Green to hang out if I want.
Re Boerum Hill, there is a huge Pathmark/Target on Atlantic Ave that I walk to when I want(I was there yesterday) and there is a grocery store also on Smith Street. Great restaurants on Smith and Court St. Brooklyn Heights is also a short walk away. Smith St has a couple of street fairs every summer(Bastille Day in July brings out loads of da wimmenz!)
There are nice/new apt bldgs on many of the side streets off of Court and Smith that you probably do not see when you walk up and down each ave. There are also some on Atlantic Ave. F train @ Bergen Street and Carroll. G also stops @ Bergen. Butcher shop also on Smith St which comes in handy when I want to grill some steaks.