It's difficult to explain the difference since those are two incomplete sentences. I've stolen a different example to demonstrate: "Consider the following sentences. Both are acceptable, but their meanings are subtly different: The books, which have red covers, are new. The books that have red covers are new. In the first sentence, the words "which have red covers" are adding information about the books. That is, they're telling you more about the books than you'd otherwise have known. (They're red, not some other colour.) All of the books are new. In the second sentence, the words "that have red covers" are limiting which books we're talking about. We're no longer talking about all the books; we're only talking about the ones with red covers. So this time, only the red books are new." So in your case, I think we should use 'that' since we're only talking about habits that you detest. I would recommend writing it this way to sound more fluent, since 'at all times' is usually included in a command or warning: Two habits that I always detest are... Hope this helps! Here's the website I used: http://www.betterwritingskills.com/tip-w022.html

The difference comes when you complete the sentence and this example is perhaps not the best way to tell you about it. Let me give you another example, The cars, which are new, cost more than 2 million The cars that are new cost more than 2 million The first sentence says that the all cars cost more than 2 million and add an extra information that they are all new The second sentence says that the cars that are new cost more than 2 million, thus limiting the cost to describe only the cars that are new. Summing up, use 'which' with commas if a group of words adds information. Use 'that' if it limits the set of things you're talking about. If you need more clarification, let me know.