I can't speak for GTP or ATB because i have never had one.
I will say that Captive born Emeralds Tree Boas are much hardier than imports and are usually less aggressive. Most of the bad stories are from fresh imports. There is nothing to watch out for when purchasing a captive born baby as long as you get it from a reputable breeder. Rico @ SignalHerp.com and Alex @ carnivorousorchid.com are the first 2 that come to mind. Now if you buy an import to save some money, that will be the first mistake.
I would never be were i am today with Amazon Basin ETB's if wouldn't have purchased my first Northern ETB. Good luck with your decision and let me know if you have any questions about them.
Ed Marino.

www.EmeraldTreeBoa.com