A very broad question: I have been offered admission to the LLM programs of both the London School of Economics and UCAL Berkeley . Berkeley wants a decison by next Monday (March 27th). I am primarily looking to work in acedemia with an emphasis on communications and IP law.
which program should I attend?

I would definitely opt for the Berkeley LL.M. programme. One of my friends attended it a few years ago and built a strong alumni network with lots of nice people from all over the world. Additionally, he has met his future wife there!
In contrast to the LSE programme, which has around 250 students, the Berkeley LL.M. is a small study course which allows you to get to know your peers and professors better. I am currently studying at the LSE and from what I can tell, the university lacks some important positive externalities (i.e. sports facilities, campus atmosphere) that Berkeley has to offer. However, if you are from the U.S., an experience in Europe might be benificial in terms of your CV and the personal experience. With regard to job prospectives in academia, I can unfortunately not recommend you where to go.