(Found it. It's in Unit 491 ("Rather: preference") of Swan's "Practical English Usage".)
Imagine a situation when two persons walk past a café. One of them notices the café, stops and says:
I'd rather like a cup of coffee now!
He is not making a choice among different kinds of drink. He merely means that it would be a great idea to go in so he could have a cup of cofee. He is not expressing his preference to coffee in comparison with red wine, say, because there's no red wine on offer at the moment.
Then our two friends enter the cafe, and he sees a wide selection of different caffeinated beverages, among them a great chocolate icecream cocktail. He says:
On the other hand, I'd rather have that chocolate icecream cocktail.
Now, he is expressing his preference to one drink among a whole range of the available items.