You could do both at the same time, for one thing. For another, communicating with the ground also has a remote chance of succeeding - if ATC knows one of the pilots is behaving erratically they can communicate with him or her on that basis, instead of inquiring about whether there is a problem with the plane. Finally, while in this case there were no survivors, in other crashes there have been survivors who died of injuries or exposure because rescue workers didn't get there in time, because they were still looking for the crash site.

http://en.wikipedia.org/wiki/Japan_Airlines_Flight_123