Green Lake yes!!
beichen is a good balance. i lived in the Green Lake/Wenlin Jie area previously and almost all the buildings are old. it's good for expat bars that close at 11.30 but that's about it. Beichen (the north) is generally quieter but it has bars (often open into the morning) and a better range of restaurants. the main advantage for me is that the mountains are 5 to 10 minutes away by bike (no traffic chaos to deal with).
@debasser I agree. I also live in the North and it's pretty good. When the subway is finished, it will be easy to go downtown.
I live in Dianchi, but as an expat I would prefer to be in BeiChen, for culture and convenience. Green Lake is too urbanised and too close to too many bars to be safe for me.