Temporary files: RAM or disk?

Obviously what I said was an oversimplification -- it's only 2 sentences. It still provides a decent mental model to answer the OP's question. If you'd like to improve on it, please do. There's plenty of room.

And, I have a reasonable view of the data centers that I've worked in... Most servers I've seen have avoided swapping. Some merely ignore it because it's redundant (Apache, nginx), and some go to unbelievable lengths to avoid it (Oracle). Very few actually embrace it (Varnish). That's just my experience. Again, if you've seen otherwise, please do share.