Nani is clearly the more talented player and has proven himself to a greater extent of being able to produce when playing in non- counter-attacking situations.
Cerci is Serie A proven and a deal for him may well provide Juve with the chance to offload some useless players.
In conclusion, it depends on the system and, if it is 4-3-3, who is the winger on the other side.
Cerci would be more suitable if Juve are just wanting an occasional option for right wing and still plan to use 3-5-2 a lot, as he's more likely to accept being rotated and has also shown himself to be capable when playing more centrally off a main striker.
But, if 4-3-3 was to be the main foramtion and if Tevez was to be the player playing on the opposite wing, I'd pick Nani. Tevez would be best playing from the left and would inevitably be inclined to drift inside and get close to the centre forward. That would leave me wanting a winger on the right to be focused on staying wider and getting crosses in and I see Nani as the more likely to do that on the right wing.
So, in conclusion, why didn't the pillock who set up the poll give us a 'it depends' option?