I also vote for Pjanic over Isco. Isco strikes me as still not the finished article and I think there are more uncertainties with him - not that I would complain if we signed him. But Pjanic has a lot of great qualities. He can be a gritty midfielder, scores goals regularly, joint leader in assists this season with Pogba, and can be deadly from dead ball situations. In a 352, I see him in Khedira's position. Having someone like Mascherano or Marchisio in the centre with Pjanic and Pogba as box to box would be truly deadly. Pjanic is also the Allegri style of trequartista - with a back 4, I would see a mid with Khedira, Mascherano/Marchisio, Pogba behind Pjanic and the 2 strikers.
The Morata buy out clause (as far as I am aware) is a 30m lump sum payment. That would enable us to foot the lump sum payment for Pjanic's buyout of 38m. And Pjanic is still 26 and is probably a few years away from his peak.