Manual hubs-
KNOW your locked, as stated
Able to do 2 low, if you keep the hubs unlocked.
More options using a spool, unlock one hub to turn.. or go 3WD to make a fun out of a smaller rig.
Downside, gotta get outside in the heat/cold/mud/snow, if you ever need it in emergency situation you cant just pop it in 4WD, gotta get out in possibly unsafe territory or no time.. (For example, chasing down a hit n run, or like in the movie Volcano.) Plus you car theives have to get out to lock the hubs if you want to out-run the police offroad =P (A plus, as long as your not the theif)
Auto hubs
Opposite of what i just said about manuals.. hoping will lock, cant use 2 low, less options for spools. Can pop in 4wd on icey roads etc.
My opinion - Automatic for daily driver/emergency vehicle, manual for trailrig/secondary vehicle.