Kinda depends on what role you want to fill in your team. If you need a sweeper, go for Zoroark, but if you need an annoyer, go for Liepard. It makes a brilliant annoyer with Prankster; it's lacking Whimiscott's Leech Seed, but the Swagger and Foul Play combo is good enough.
Liepard @ Leftovers
EVs: 252 HP, 252 Def, 4 Sp. Def
Nature: Calm/ Impish
- Thunder Wave/ Encore
- Foul Play
However, Liepard is lacking in any offensive presence, and is dealt with by a Mach Punch form Technitop. Still, its substitute can be hard to break, and used correctly, Liepard has the ability to KO an entire team just through haxing.
For Zoroark, you have the option of either a choiced sweeper or a nasty plot sweeper. Zoroark will deal actual damage, rather than troll with with Prankster.
Zoroark @ Choice Scarf/ Life Orb/ Leftovers
EVs: 252 Sp. Atk, 252 Speed, 4 HP
Nature: Modest/ Timid
- Dark Pulse/ Night Daze
- U-Turn/ Nasty Plot
OR (physical set)
@ Choice Scarf
EVs: 252 Atk, 252 Speed, 4 HP
Nature: Jolly/ Adamant
- Foul Play
- Low Kick
Zoroark has the option of being either a physical or special Pokemon. While many people use it for its base 120 Sp.Atk, its 105 Atk shouldn't be overlooked.
Punishment is a better move than it seems, what with calm mind stallers so prevalent in UU/ OU. Foul Play is brilliant for Pokemon like Choice Moxie Salamence, which will continue to sweep your team unless you have something that is immune to its move. Its physical set is usually for clean up/ late game/ counter sweeping. Zoroark also makes a lead, being to scout and U-Turn out.
Conclusion: if you want a good dark sweeper, go for Zoroark. If you already have the sweeping roles filled and the coverage you need on your team, go for Liepard. People think that Liepard is just another NU Pokemon, but it can function just as well as Whimsicott in stalling/ annoying your opponent.
I don't know what the rest of your team is, so I can't exactly say "xxx will be better for your team". However if this is in game, use your Zoroark, since Prankster is a DW ability for Liepard, and Zorua is easily obtained in Driftveil City.