In BE, which can be used in a restrictive or a non-restrictive clause, but that can only be used in a restrictive clause. To me, there's no difference between your examples a&b. The stress would be dependent on context and intended meaning, not automatic. To distinguish between which + restrive\non-restrictive clause in speech would be done by a short pause or lower stress to show it's non-essential with a non-restrictive clause.
I understand the position in AE is different and, I presume, Canadian english follows the AE pattern.
- For Teachers