Just got a shiny zoroark and I got lucky with the rash nature and I was wondering if I should use a life orb or choice specs for him. His move set would be
•nasty plot/shadow ball (nasty plot if life orb is better or shadow ball if specs are better.)
I would say Choice Specs.
Choice Specs increases the power of your special moves by 50%, but only allows you to use one attack. The Life Orb only increases the power your moves by 30%, takes away 10% of your HP each time you use an attacking move. The Choice Specs cost is much lower than the cost of Life Orb, so I would go with Choice Specs.
Hope this helps!