Thought it time I introduced the 'dawg'. This is Zudnik:
He was a wolfdog, a hybrid. His mother was a malamute; the father was a wolf. Few people think hybridization of canines is a good idea. I agree. Zudnik came to us as a gift, and we had no heart for putting him down. We dealt with him. Loving, ferocious, gentle, vicious, he spent ten good years with us. He was a beast, but he was my beast.
DAWG SEZ 3:
The use of that or which often bamboozles the sleepin' dog. Seems normal to say 'This is the fellow that stole my car,' or 'it's the kind of weather that I like.' That is what we say, but which is what we write.
Why? Go figure. Some think it's because words like who and which hark back to Latin pronouns, and so folks have the notion that those words are more refined, more literary. Others suggest that the clause begun with which is rare in speech, but more common in the written word.
So what's the difference? Fairly straightforward. Use that if what follows is limiting or defining. Our first example is just such a construction. Use which when the following clause can be left out like a parenthetical remark: 'The Columbia River, which is controlled by several dams, flows into the Pacific.'
Do what comes naturally (what'd ya expect from a dog?). Would you say 'This is the hombre which copped my ride.' I think not. You might write 'The Columbia River, which is controlled ...'; but you'd say 'The river that's controlled by dams is the Columbia.'