Recommending Stellvia and Scrapped Princess for each other may seem a bit far-fetched, one being pure sci-fi and the other being mostly fantasy. Nevertheless, both have a very similiar feeling, especially when it comes to cuteness factor, mix between humour and seriousness and, of course, the oh-no-I-can't-sleep-yet-gotta-watch-just-one-more-ep-feeling.
The main characters are also a bit similiar - Shipon and Pacifica are both special but neither wanted fame or adventure. The sidekicks are also very nice in both series, Stellvia having Shipon's classmates and Scrapped Princess having Pacifica's siblings and friends.
In conclusion, both series are very nice, lighthearted stories about meeting challenges with the help of friends. If you liked one, you will very probably enjoy the other.