caeruleancentaur wrote:
>The red flower is on the table.
>The flower, which is red, is on the table.
>It seems to me that these two sentences are semantically the same. I guess 
>it's up to the speaker's discretion which one to use.

If I restrict myself to sentences which seem plausible as spontaneous 
utterances in conversation, I can't think of one containing "which is red" 
in this context. (Asterisks indicate IMHO implausible utterances.)

- There's a red flower on the table.
- *There's a flower which is red on the table.
- What have you put on the table? / The red flower. [but not the blue one]
- What have you put on the table? / *The flower which is red.
- The red flower's on the table and the blue ones are on the shelf.
- The red flower's on the table and the ferns are on the shelf.
- *The flower which is red is on the table and the ferns...

A "which" clause in English is much more likely to involve a content-ful 
verb, a full NP or AP, an object-relative clause, or a combination of these. 
For example:

- The flower [which] I bought several weeks ago is still on the table.
- The flower, which is several weeks old, is still on the table.
- The flower which you liked is on the table.

In any of these, the relative clause could be restrictive in meaning, and in 
the first two at least it might also not be. I agree that, as a rule of 
thumb, a "which" clause is more likely to be non-restrictive than an 
attributive adjective, but cf. for example: "Look at all these great 
books!", where "great" is plausibly (if not obligatorily) non-restrictive.

I think that the difference between the two constructions is determined by 
the syntax of the predicate ("red", "you liked [it]"), and only secondarily 
if at all by the restrictive / non-restrictive distinction.

Jonathan.

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