Ok so you're Williams...
You are unhappy with the customer deal you have with Toyota and are looking for a change.
The Merc, BMW and Ferrari engines are not available.
This leaves the Renault, Toyota and Cosworth engines.
Which do you choose.
Do you pick the Renault as it's proven in the Renault chassis at least (RBR build fragile cars which I'm sure has had some bearing on the engine failures they have had) and has decent if unremarkable power and good heat characteristics like the Merc...
Or
Do you go back to Toyota as is in all likely hood they will be able to make some changes to their powerunit to bring it upto spec (and it has been incredibly reliable). The risk being that they may want Williams to keep the slow Kazuki in a chair...
Or
Do you go for the unknown Cosworth as they going to be able to heavily homologate the CA2006 into the CA2010 and could easily spring a surprise which might be worth the risk...the risk being unreliability...they are also likely to be easier to work with and don't have a young driver they can force on the team.