The series 10 has a larger screen in a smaller overall package. I think it has a wider screen that goes close to the edges. The Ultra has better battery life.Hmmm, I’ll have to look at that. Edit, from Apple.
Ultra 2
49mm
410 by 502 pixels
1185 sq mm display area
Series 10
46mm
416 by 496 pixels
1220 sq mm display area
Remind you I’m coming from a series 4, so either ultra 2 or series 10 is a good upgrade for me.