I actually think a spherical shock wave (for the Death Star) would look better regardless of the science also a ROTJ style momentary egg crack before the big boom would be nice.
Alderaan should have a different effect. I rather like the idea of it seeming as if it be slammed by the beam and shatters like a conker.
The Death Star is blowing up from the inside where as Alderaan is being smashed from the outside.