I'm no expert on either of the green tree snake species but the experts I have asked have recommended a smaller cage for babies, one reason being in a cage that large the baby may not be able to find food. I thought about a misting system for my emerald but with as much cleaning as you'll have to do, it may turn into a complex system that is more of a pain than help. If you don't have a
GTP or an
ETB and this is your first, I'd keep the first enclosure simple. Not to say you haven't done your research but I'd build a simple cage that works before building one with bells and whistles. If something goes wrong and your temp/humidity fluctuates outside ranges you want, both animals are more unforgiving than other species of snake.
I built 2 of the exact same cage buying opaque rubbermaid bins from wal-mart. Total cost for BOTH was around 100 bucks. It makes for easy cleanup. I remove the perch the snake is on and put it into the clean cage, that way I can clean the unoccupied one without any problems. Half the time I don't think she knows she's in a new home.
To answer the last question, I prefer
ETB's. I like the white diamonds on their backs. Doubtful that I will but if I decide to breed them one day,
ETB's have live babies...another plus because you don't have to worry about taking care of eggs.