Question: I know that progressive jpegs download in multiple passes, and optimized are smaller (and thus faster to finish downloading), but does anyone have any hard data or best practices on *which type users actually prefer*?

Answer: 
Here's an alternative perspective on it. Not an absolute rule, but a thing to keep in mind.

**It depends on what the image is for. Content, or decoration?**

If it's a background or other design element, use progressive, so the page looks okay-ish as soon as possible. If it's an element that moves stuff around, it needs to be there ASAP so people don't mis-press a button due to layout shifting. (been there a couple of times on mobile.)

If it's content the user is going to download, or consciously pay attention to, use regular JPGs. This communicates to the user that it's not yet finished, that they have to wait a bit. This stops them from saving an unfinished download, and they won't experience the site as bad quality but the server/connection.
I'm having trouble finding a definitive, recent answer but from what I can tell, Mobile Safari, to pick just one mobile browser, supports progressive JPGs, but doesn't actually load them progressively--meaning it just waits for it to download entirely, then displays it.

So it seems that the visual progressive loading feature simply isn't all that relative today, making the benefit much less appealing.

That said, making a JPG progressive *may* help compress it's total size, so by all means, that's still a benefit, but it's probably just not a big enough benefit where it's caught on widely. 

Also, as stated in my comment, I don't think this is a 'vs.' issue. Unless I'm misunderstanding what you mean by 'optimized' a JPG can be both progressive *and* optimized. In fact, any image format for your web site should be optimized.