I'd pick Pjanic over Isco for the same money. I do rate Isco higher as a CAM especially in terms of potential, but I can't see Isco playing in the Marchisio role at all which is our most urgent gap now. Pjanic can also play next to marchisio to rest Pogba in some games without disrupting our balance the way Isco would.
I pick Pjanic for his versatility and also for the fact that he is a Roma player. I would very much like to set a precedent of buying our competition's best players a la Bayern. This summer we could start with Pjanic and that can break the unwritten rule that stops us from buying Roma and Napoli's best players. Perhaps after Pjanic, more players in the future who would fear the backlash from a Juve move, would be more encouraged to join us and would see it as normal. For Bayern it started with Gotze, then Lewandowski made the same switch and now possibly Hummels too. I would very much like players to see Roma, Napoli, Milan, Inter etc... as stepping stones for getting into Juve's squad. This is really a bridge worth building.
Eitherway I am not worried, Beppe is probably following through on multiple scenarios and is probably talking to many candidates not just Pjanic and Isco.