I have noticed that it seems to be a British tendency to use "which" instead of "that", but I've seen and heard Americans use them interchangeably too. In informal English (especially spoken), many use either which in lieu of that regardless of circumstance, and it is considered acceptable to do so in most instances. In more formal encyclopedic prose though, I think the grammatical precision is necessary. I hope someone who has more knowledge in this area can help me out here.