check out the american serama thread, the malaysian serama thread and the japanese bantam threads... also, google for breeders or standards of perfection for both breeds... way different origin and breed all together...
About the only thing that is the same between the two is the tail carriage. Japanese are bigger, have a horizontal body and wing carriage, and should have shorter legs. Serama are smaller and have an upright body carriage with a vertical wing carriage. There are several other differences but these are the most notable.
The Japanese Bantam was a large part in the making of Serama, but they are two separate breeds. The serama are smaller with a full upright chest, vertical wings and tail, with a good, muscular and lengthy leg to carry the wing vertically. A proper quality Japanese Bantam should be around 20-24 ounces, short leg with horizontally held wings and a vertical tail. I was taught if you can see the leg on a jap, there too long!