Johnny Hargreaves

Yes, obviously they are nearly always interchangable...It really is only a question of style.
There are a few extra guidelines I know of:

Guideline 1: "Of" is more formal, "'s" is more informal. In a news article for example, there'll be a lot more "of"s while "'s" is used more in informal letters and casual conversation. Both sentences seem good to me in the right context. (The second seems kind of doubtful to me, but it's passable.)

Guideline 2: "Keep it short". "The bow of the ship" is 5 syllables, "ship's bow" is only 2.

Guideline 3: If you used "of" already, use "'s" and vica versa. ("The name of John's cat.")

I think this one is incorrect. Not only is milk innumerable but it's also used in the "idea" of milk sense here. It definitely should be ''of'.

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