Question: What is the difference between these two sentences below in terms of semantics or for that matter any other aspects? 

> 
> He is a hard-working man.   
> 
>  He is a man who works very hard. 
> 
> 
> 

Is the distinction simply a matter of style? If not, what differences do they have in respect of giving information about the noun they modify?  

To be more specific, I asked this question because I sometimes can't decide whether to use a relative clause or an adjective-like phrase to modify a noun.

Answer: 
Both convey the same information, though in my experience, the first sentence would be the way that idea would be more commonly phrased.
> 
> He is a hard-working man.
> 
> 
> 

This sentence describes the man himself. Hard-working is an adjective phrase. I would expect the man always works hard because it is a description of the man himself.

> 
> He is a man who works very hard. 
> 
> 
> 

This sentence describes how he works. 'Very hard' is an adverb phrase. Because the phrase is an adverb it could be qualified like, "He is a man who works very hard one day a year." or "He is a man who works very hard at avoiding work."

While they are similar, they are not the same.