According to an article in The Times on Wednesday (28th May) organic milk is healthier. The milk tested from cows fed on organic diets contained more good fatty acids (the level was up to 60 per cent higher in the Summer months) when compared with cows fed in fields where fertilisers are used. The difference is thought in part to the higher levels of clover in the diet of the organic cows. This is leading to research to improve the health qualities of butter and cheese.
Having switched to organic milk about a year ago (although Waitrose do have a tendency to run out from time to time) I can definitely say that not only is it better for you but it also tastes better and is creamier too.