Hello Hazman;
The tree climbing harness has more padding under your legs that allow you to seat down for longer periods of time since that is what you mostly do while tree climbing anyway.
The rock climbing harness will cut the circulation to your legs and pinch you in the wrong places. (Ouch). Also a good quality Tree climbing harness is most likely to be rated to meet ANSI Standards for Safety gear that most basic rock climbing harness may not be.
You can climb on almost any harness but the comfort of a properly sized Arborist or Recreational Tree Climbing harness is worth the extra money. Read some of the de_script_ions in the gear review area of this web site for some suggestions. Also visit the New tribe web site for photos and uses of different models.