sounds way to young to choose one to me.
i have black tailed white japanese as well, at 6 months i am just starting to see what their tails are going to look like as adults, i have 3 roos (of 8) and can tell #1 has a squirrel tail (not good), #2s tail is looking quite nice (aside from not yet having the few very long tail feathers) and #3 has moulted his tail so we shall see in a few weeks what comes back in!
i couldnt have told any younger which was nicest (never had young japs before though), im leaning towards roo #2, well see.