As much I enjoyed Boxers, I think Saints hit me a lot more. It's significantly shorter, but it packs even more of a punch right in the soul, as it wraps up all ambiguities left from Boxers. It may be because Boxers starts covered in idealism and then starts tarnishing that pedestal as the boxers encroach upon Peking, and Saints starts at the absolute bottom and then gifts us with a hope we know is futile straight from the beginning.
Overall, Boxers and Saints is a very well written and meaningful duality of a horrible conflict in Chinese history. We are close to the main characters in both books, but neither the Christians (mainly the foreign missionaries and army personnel) nor the boxers are deemed as heroes or villains, which was devastating in Boxers in our desire for Bao to stand as the hero, and then just a fuel for more excruciating conflict in Saints when Vibiana is already surrounded by so much of the same.
The art of Saints also helps it stand out, particularly in the nature of its sepia tone. The way that Joan pops out as the color and the hope of Vibiana's life strikes the reader, though the nature of Joan's own end implies much the same about Vibiana's, increasing the sense of foreboding left behind from Boxers.
I really liked the door both Boxers and Saints left open in regards to both of their visions. Were Bao and the disciples really possessed with ancient Chinese gods? Did Vibiana witness the rise and fall of Joan of Arc in parallel to her own life? One can't really say; both of the most overt examples of these fantasies (Bao with the man on the mountain, Four-Girl partaking of the devil raccoon's flesh as offered to her by Joan) are done when they are alone. We never know if they are real, though it never removes the characters of their agency. It adds an extra layer of thought.
If you're not into graphic novels or comics, I'd give this a try right up there with Maus and similar titles. It's worth a read.