The mizuna is on top, with the jagged, pointy leaves, and the tat soi is on the bottom.
I've read some more about them both on the internet. I found mizuna described as sweet, earthy, tangy, mild, and peppery. One website, Harvest to Table, said, "You can toss young mizuna leaves—which are mild tasting--in a mixed salad. Larger leaves—which can have a mustardy or bitter-green tang--are best cooked briefly." A cookbook I have from the library, How to Eat Supper, includes "A Taste Guide to Salad Greens," and classifies baby mizuna under "Standouts but Not Overpowering," while it puts mature mizuna in the "Greens with Punch or Bite" category.