Hmmm, tough question.
Diego is perfect for this Inter. The missing link between the midfield and the attack.
But tough to say no to Ribery, a player who can also make things happen when nothing works for the team, who can make even Inter's left side super strong. With Ribery on the left, Maicon on the right and Zlatan up front...
I'd still choose Diego if i was Mourinho. It's important for them to keep Zlatan as the main figure and the biggest ego in the team, which might be a problem with Ribery in the team also, and i'd try to keep the 4-3-1-2, where that guy behind the attackers is a magician with the ball, instead of hard-worker.
If they sell Zlatan though, i hope they get Ribery, not Diego, and it's from purely egoistic reasons. Juve has no chance to get Ribery anyway, while we can get Diego. If Inter are after Diego too, then we're screwed.