I don't know what your niche is but I've been running AdSense since the beginning. I also was running Kontera for years until earlier this year when I switched to Vibrant.
When you say one year contract, I'm pretty sure that just means that the contract is valid for one year. It doesn't mean you have to leave their code on your site during that time. Check out what termination clauses are there. Usually you have to give a company 30 days' notice if you're going to remove the code.
This may be as simple as a misunderstanding. I would be very surprised if the contract states that you cannot get out of the contract with X days' notice. I think it was Kontera that said I had to give them 90 days' notice but I wouldn't sign unless they changed it to 30 days' notice. Just look at the details... it sounds though that they might not have sent you an official contract for signature yet... that would be the next step, then you can clearly evaluate the terms, and negotiate as needed.
In my niche Vibrant makes more money than Kontera, considerably more. Keep looking into the issue, don't just brush it off. I have noticed also that whatever in-text ad company I go with it has very little to no impact on AdSense earnings.