Onyx clowns are true perculas; their genetic stock comes from around the Solomon islands and so they exhibit more melanism (turn black), but they are perculas. In the tank their behavior would be the same.
Well, if what you mean is should you get the orange or black variety, I say go with the black. The orange ones are a bit over-done, IMHO. The black ones are a nice change while still looking like a clown fish.
Think you are better off with a young female and a juvenile male, and that the female is the occ. Thing about perculas is unlike most clowns they don't live in family groups, they live in pairs. So IMO its better off with a female occ as they are more accepting of others, and as she is already a she they don't have to squabble over that.
Just started the process at work today, pairing a juvie male perc to a young female BW occ. Was ready with the net so he would not get tore up if she was aggressive. Sometimes you have to switch out males until you find one that she will accept - who knows what she judges on.