That's a tough choice! I have been meaning to subscribe to Hip Mama for a while now. I wouldn't say it's aimed at younger readers, it's just pretty subversive and probably at times there have been contributions by younger writers. It varies quite a bit on content issue to issue though. I really like it.
Brain, Child is really great though too. You would probably get more actual reading material out of Brain, Child. It has a few more pages I think. BlissfulMama, Brain, Child is basically like the New Yorker but all essays about parenting. It's really good.
I say subscribe to both! I'm no help am I?
