IMO its really all up to you (though i'd got with an ETB)
ETBs and GTPs are so crazily similar...
I'd always suggest starting with a young neonate. The ontogenic color change that comes with these animals is nothing short of stunning, and you'd hate to miss it.
I tend to side with ETBs because they are larger animals. I know a hatchling GTP couldn't suck down a rat pink like ETBs can.
Caging is also up to you. Keep in mind, big cages with small animals usually gives you nervous animals, so caging will have to change with time. In my experience you can get away with 2 cages for the entire lifespan of an ETB. Starting with something equivallant to a 20gallon tank for the first year and a half to 2 years, then up to a larger Vision 222 sized enclosure.
I don't think misting systems are required for just one animal. Now, if you're breeding, then thats a different story. But if you must, BeanFarm does sell a great 4-nozzle misting system. Just cap off 2 or 3 of the nozzles and you'll be fine. Set the system to spray at 8am and 8pm for 3 minutes.
I think with time you'll come to realize that keeping ETBs and GTPs are just like keeping any other neotropical snake (like Brazilian Rainbows).
for more husbandry info I'd suggest checking out the following:
http://www.emeraldsoftheshield.com/
http://www.amazonbasins.com/
as for breeders
Definately check with Rico Walder at
http://www.signalherp.com
he does both ETBs and GTPs an is a GREAT guy that can also answer ANY question you have for him...