To resolve all this argument, there's an arbitrary rule that people don't actually follow stating that restrictive relatives (like the topic title) should use "that," and non-restrictive relatives should use "which." So yeah, that's kind of what Goafer is saying, as non-restrictive clauses are normally separated by commas. But in reality, it's as much a "rule" as the idea that one shouldn't split infinitives or end sentences with prepositions - speakers use both "which" and "that" somewhat interchangeably. That said, it seems right for me to use "that" here, and I'd also contract the "is." Still, it may well be that Chair's intuitions differ to mine.
And now back to the thread...