The 13" MBP that is most comparable to the X1 Carbon also uses U series, while the 15" MBP is best compared to the X1 Extreme, which, you guessed it, offers the better H series CPUs.
You can’t have your pie and eat it, in exchange for battery life having U is completely understandable. Depend on your workload (mostly just a ssh frontend), even Y is acceptable.
Same for Dell XPS 13 vs 15.