I feel he should go really, Man United would be stupid not to cash in on him now.
Theres a chance that he could have a really poor season next season, or even worse get an injury @ the worse end like Eduardos. If any of these two happen you can bet your money on his valuation being dropped like s*it on a shuvvle (sp). And when he definately wants to leave next year United will have to let him go for a reduced rate.
This has been his only good season for United since joining. People forget hes been @ United for 3 Years or so, out of them 3 years hes had one really good season.
Defenders could suss him next year as Essien did the 2nd half and the goals and the step overs may not come as easy...
I could be wrong though he could be worth even more money next year, i doubt it though, that rarely happens in football. And they will know he is going next season regardless of what happens.