The shows have a lot of similarities: great animation, reverse harem theme, motivational and feel-good tone to each episode, overall light and positive vibes. I guess if you want to be cynical you can say both shows follow the same 'script'. Personally I enjoyed them both and I think if you like one you'll definitely like the other.
-Both are reverse Harems
-Both Main Characters are girls that are surrounded by very talented boys
-Both MCs are talented
-Both MCs bring the boys together
-Both MCs are loved by all the boys in the groups
-Both animes take place in a school setting
Both have the same reverse harem, where it involves one girl with a bunch of guys, vying for the girl's affection, however, characterwise, the girl in Magic-Kyun is a bit more matured compared to the girl from Watashi ga Motete, whose is a bit on the immature side.
The similarities are obvious once you begin watching them; musical, reverse harem, group bonding, and school life are a few similarities that stand out the most. The biggest difference in Starmyu is the absence of a female main character, Starmyu's cast is entirely made up of guys. Other than that their really isn't that big of a difference except the lack of romance replaced in Starmyu by a little bromance but with an all male cast what do you expect.