ResearchGate is very nice, but you have to request and wait for the full-text of a publication from the author. In Academia.edu, you just get a direct link to a full-text source or a straight up PDF download. At least that is how my experience has been so far.
Another thing I noticed is that most of my professors and colleagues seem to have active ResearchGate profiles. Academia.edu is still pretty new, so I only found a few friends that had Academia.edu accounts.
It will be interesting to see where this all goes. I will personally be using both sites, but I better get back to finishing my first publication :)
