I'm actually in the process of making the same decision. I keep going back and forth and can't seem to make up my mind. From the research I've done and people I've talked to, I eliminated the ETB. I'm debating between a GTP and ATB. You should also take a look at the ATB for your first arboreal snake, they seem to be the most forgiving of minor husbandry mistakes. If I do go with a GTP, I'm gonna go with a yearling or adult.

Keep us updated on whatever you decide and post some pics! Good luck!