According to SkyF1, it’s apparently linked to what engine manufacturer to go with from 2013. If they go with Bianchi, then they’re committed to Ferrari customer engines as Bianchi comes from the Ferrari fold. If they go Sutil, who comes from the Mercedes stable, they will commit to Mercedes engines. This is their dilemma and why it’s taking so long.
From a personal perspective, I think they should try new blood, Sutil has had his chance, in fact many of them. Another consideration will of course be how much sponsorship each driver brings to the team.