When I first started collecting arboreals, I jumped right into buying a couple pairs of chondros and ETB's. I agree with Bill in that chondros are easier to keep of the two. Everything has to be just right for ETB's to thrive. Chondros require specific environmental conditions as well though but are more forgiving of "slight errors".
Although I initially shunned them off, I recently began acquiring ATB's and must say that I am quite pleased with them. I have heard that these are even more forgiving of husbandry errors than the other two but I am not about to test that theory.
My personal opinion would to not acquire an ETB as your first arboreal. ATB's are very hardy and relatively inexpensive compared to the other two, so they may make a good starter animal for you. Chondros will always be my favorite though so I would never try to disuade someone from going that direction. Assuming, of course, you do your research.
Good luck in whatever you decide.
Bart