The ThinkPad Twist rotating display mechanism has been around for a decade now, and it's tried and true. It is however bulky by today's standards (the mechanism, not the Twist), so manufacturers are now moving away from it given the drive to make wafer thin portables. It works well and is robust. I'm actually sad to see it go.
With the Dell you get SSD drives, while that's an upgrade option for the Twist, which explains some of the price difference. The rest is the full HD display on the Dell XPS 12 vs. the 1366 x 768 resolution on the Twist. With both equipped with SSD drives and the same Core i5 CPU, you'll see very similar performance. They're running in nearly identical hardware inside.