These fish are not very closely related. The Asian red tail is
Hemibagrus wyckioides, which is considered to be the most aggressive freshwater fish in the world. It is a dark grey color with a vivid red tail and can get to be about three feet long. These fish are virtual half-brothers to the Black Devil Catfish,
Hemibagrus wyckii. This is not a fish you can keep with other fish, they are so aggressive they will attack anything in their tank; other fish, heaters, filter heads, your hand, ect.
The South American red tail is
Phractocephalus hemioliopterus. The South American variant is a very common fish in the aquarium trade, and is not very aggressive. It will eat anything that fits in it's mouth, but it won't dismember it's tankmates for the fun of it like the Hemibagrus. If kept with similar sized fish (five feet long in the wild!), it's an OK community fish. This fish is also much more vividly colored, they're black and white with a rust or orangish colored tail. I've seen some fish that where brown with white markings, but I've had two of these now and both of mine colored up really well when fed on Hikari.
I currently have one of the SA ones that my wife named Oreo.