There is no doubt at all that Modric is a much better player than Marchisio, and if people remember my posts and opinions on Marchisio, they'd know i rate him.
They are comparable as CMs, who aren't DMs. In Juve's 4-4-2 those two would fight for one spot in the team. That means that they are comparable.
Modric is probably more comparable to Aquilani than Marchisio, but Modric's work rate and movement puts him somewhere between the two Juventini and you can compare him to both of them.