Total Recommendations: 3
As of right now, they're both still publishing, and one only has 2 volumes under its belt, but I've noticed some similar elements. They each have that royal aesthetic, following heirs to a kingdom in a world hasn't quite caught up to real-life technology. They also each focus more on the technical/political aspects of running a kingdom than your typical fantasy. (Which is the main thing that made one remind me of the other.)
It might be quicker to list the things these shows DON'T have in common. Similarities include: A strong comedic element Lots of otaku culture references The characters are all a little deeper than first meets the eye Funimation gave each of them a really good dub, if you watch anime in English Basically, they're each a break-down of the reverse harem genre, spurred on by a main heroine who doesn't really fit the role. If you enjoyed one, I'm sure you'll like the other.
Two stories about overly cautious young girls growing up and learning to be brave through their encounters with spirits and mythical creatures. Spirited Away has more of an immersion into the spirit side of things. A Letter to Momo focuses more on the family aspect. But I definitely saw similarities between Momo and Chihiro-- the two protagonists.