I'd give Hulkenberg the seat. He's shown he can be quick. Quicker than di Resta.
However, it's generally-accepted that Sebastian Vettel has a pre-contract in place with Ferrari for 2014. In that case, no driver is going to want to commit to 2013 and 2013 only, unless they think they can do such a good job that they convince Ferrari to invest in them rather than Vettel. And every driver knows that that will be a tall order, so nobody is going to be stupid enough to try. Except maybe di Resta. Given all his big talk and the hype surrounding him, he'd be arrogant enough to think that he could pull it off - right up to the moment when Ferrari decide to take Vettel, anyway.
In other news, Schumacher has reportedly been in talks with Sauber management, and Peter Sauber himself has said that he would gladly take Schumacher if he felt Schumacher was the right driver for the team. Honestly, I don’t think there is a place for Schumacher in the sport anymore. His return hasn’t gone according to plan, and he’s been upstaged by both younger drivers and other returning drivers. What he has achieved has been great, but now he’s just treading water. I’d much prefer to see Sauber take on a promising young talent, like Frijns, da Costa, Evans or Wickens, rather than taken on Schumacher.
I think Petrov's contract at Caterham is for 2012 only, and with his sponsors, he is also a Sauber candidate I would say.
Petrov doesn't have sponsors. The Russian government pulled his funding, and Charles Pic is believed to be moving to Caterham next year. His best hope is to count on Kovalainen moving elsewhere in 2013 and that Marussia finish ahead of Caterham; that way, Pic would be replacing him rather than Petrov. And since it would be difficult enough to catch up to the others on a reduced budget as is, Caterham would be mad to replace both drivers for 2013.
It's either that or hope that Marussia get 10th in 2012, and decide that they don't need a pay driver like Pic or Max Chilton, and instead decide that a Russian driver would be worth a lot more to them. Or hope that Timo Glock loses patience with the team, so that they can get Chilton's money and the profile of a Russian driver in a Russian team.