GT have a bold outline of orange, red, or white on their caudal fin. Blue acaras, only when their colors are good, have a deep maroon-red border that fades into the caudal fin. That's the main difference. Green terror's bodies also have a green/yellow sheen (def?) rather than blue/green/yellow speckles. Green terrors also have a bigger forehead than blue acaras. Blue Acaras are much darker than green terrors. When they are juvies, they both look identicle, besides the caudal fin thing.
if you are getting one of the two, go for acaras.