Well, Nobody said my opinion here.
I always use both.
My Classic set for my lucario is..
HP Rock or Dark Pulse
Now my explanation.
Nasty Plot is amazing on lucario, but the real delima here, is its hard to live without both fighting moves, in this case, its easiest to just use both.
I've had several moments where I was sweeping, and I had to alternate between both of them.
So I find it usefull to use both.