Does AdSense optimize ad units? If a particular spot performs better with image ads than text ads, does it show these more often? Or if a particular spot performs better with multiple ads than a single ad taking up the entire spot (even if it pays a higher CPC), does it do this more often?

Dani - these questions are DEEP!
Google says... "Both text and image ads will compete in the same auction to display on your pages, and we'll automatically display the ad(s) that we expect to earn you the most."
So it does sound like they optimize ad units based on performance but it also sounds like other variables are taken into account.. Like the number of relevant AdWords advertiser(s) that are in the auction? How much they are willing to spend? If they targeted your site specifically?

Google has very in depth research and knows which ads work best on what pages... some images, some text.

All depends on the niche of your topic and the pool of advertisers.

I would rather have straight text because it doesn't clash with the rest of the page design if given the choice.... but most of my sites sell ad space privately.