To be honest, the SJD/JSD (the real legal doctoral degree) is purely a sign of someone who wasn't good enough at writing/publishing to get into legal academia without one, so it's actually more embarrassing than impressive. Most legal academics don't have one (in contrast to a PhD in other fields) because it is largely a waste of time.
JDs are not really doctoral degrees to begin with, as any lawyer with any degree of humility will admit. (I have one.)
A JD/PhD is becoming fairly common for legal academics. And, as another poster said, there's nothing particularly impressive about an SJD.
What are the most common PhD’s for Lawyers if they don’t pursue an SJD? Political Science? Philosophy? I know Oxford has a DPhil in Law. Yale has a PhD in Law as well as a SJD. Wouldn’t those two be the same?
I'm not sure what the most common JD/PhDs are, but I had a couple professors who had a JD/PhD in political science, so I do think those are fairly common.
The distinction between an SJD and PhD differs from school to school. My understanding is that a lot of times the SJD is specifically geared for those who want to teach law school, and other times the SJD is targeted at those who originally received a law degree from a non-US school.