I think Andy has better chances to win at Wimbledon. Nole in his current form and peak Rafa are better HC players than Andy, and also Roger can still be just as good as Andy on HCs. Of those players, grass favours only Roger, so I believe Andy has a better chance to beat Nole/Rafa on grass.
Also, DelPo is also a serious threat for Andy on HCs but he's not too impressive on grass. On grass, there might be even more serious threats like Tsonga and Berdych, on the other hand they are so inconsistent that Andy would still be the favourite against them.
And, I think home crowd at Wimbledon is a huge help for Andy. AO final showed he isn't at his best in GS finals, yet he played very well in Wimby final until getting broken in 2nd set at 5*-6, which destroyed him.