Was never a Jag, but served as both enlisted and officer, both active and reserve.
<<lifestyle>> JAG officers, like any officers, will probably have to relocate periodically as part of their career. Civilians probably do not. Fitness is part of your professional responsibility in the military. It is not a consideration as a civilian. You get 30 days paid vacation in the military and your health insurace is essentially free. Not so for civilians.
<<areas of practice - are there any specializations? ex: tax>>
Can't really speak authoritatively on this, but my inclination is to say that such opportunities are by far the exception.
<<* payment and compensation(s)>>
Do a google search on "military pay". Typically, you will make O-3 (captain in Army, Marines or AF or lieutenant in the Navy or CG) after 3 or 4 years. You also will receive a considerable allowance for housing and food. (BAH and BAS). You may also be eligible for things like hazardous duty pay, combat pay, parachute pay, sea-duty pay, family separation allowance, etc. As a rule of thumb, I'd say add another 30-40% on top of the base pay for allowances and incentive pays.
If you do 20 years, you could retire before you turn 50. You could have a substantial lifetime stipend as well as lifetime health benefits.
<<transition to civilian firms after service (best areas of law after JAG)>>
Possible, but again, my feeling is that this is the exception, not the rule. Chances are you'll have to start again near the bottom rungs of the ladder.
I think I'd look at it this way: 20% of attorneys make crazy cash.
Those guys will absolutely be much better off, financially, to go civilian versus JAG.
80% of attorneys probably make a comfortable, though not great living.
JAG attorneys are probably way ahead of those attorneys.