Man they are both very very cool. Its hard to know..
I have never owned a GTP but I have like 25 Amazon Basin ETBs. I must say I really like them because I find them very easy to care for, especially to keep clean when comparing to terrestrial species like Boa constrictor or Epicrates cenchria.
Once you have a well started specimen (CBB), and your terrarium set up is ready with all the temp/ventilation/humidity controled it is very easy to keep the snakes. Its just more expensive to make the terrarium, but once its ready they are very low maintenance snakes.
From the little I know about GTPs they sound like a little more difficult to keep, as the feeding/defecation scheudle has to be more closely monitored as you would do for an ETB.
As for aesthetical purposes, I really love the strong yellow belly of the Amazon Basin Emerald, with deep green that creates a beautiful contrast with the white pattern.
On the other hand GTPs come in many morphs and you have many many choices. From bluish, green, yellow to albinos.
On both species there are choices, limited by the amount of money you want to spend.
I would not worry about the heat pits in the lips of the emeralds as this makes them even cooler snakes once you see how sensitive the snake is to heat prints at night. Some emeralds are really curious and come to the cage´s glass to "see" you once you come close.
There also the reproductive side.. GTPs obviously breed much more, being oviparous. Female ETBs on the other hand are bienal breeders, as they will have to carry the babies for up to 6 months and need a whole year to rest and gain weight in order to breed again.