Kaitou Saint Tail

Both are about stealing and love. 

this is great and has the same genre romance=check magic=check and has the same children theme 

Cat's Eye is also about thieves and romance. In This case, it's three sisters who are thieves. They also leave a calling card at the scene of each theft. One of them is dating the inspector who is assigned to capture them. 

they look the same!! 

Both are atypical of their respective genres as they use unconventional, non-violent means for saving the day, eg GaoGaiGar uses environment altering power tools, Saint Tail uses stage magic like. Both feature long haired, redheaded mains.Are very optimistic and lighthearted in tone. Both have a very classical symphony soundtrack. They also have religious themes or terms that play a role in the story or events, such as Meimi praying before becoming St Tail and GaoGaiGar's Hell and Heaven involving a prayer like hand clasping. Both heroes need the assistance of a female who waits in a bulding in order to turn into their titular form/persona Both Harken back to ideas and themes of an older era of their respective genre whilst also updating said ideas and themes for the 90's, especially after a gamechanger. For Saint Tail it is pre-Sailor Moon Magical Girl's, for GaoGaiGar it is pre Evangelion/Gundam for Mecha. Lastly, both were in the process of being dubbed in their home video release, but said dub was cut short and instead switched to sub releases. read more 

Isn't it obivous? Both are kaitou shows :D! 

-Overall plot is almost exactly the same. -Both involve stealing -Both involve disguising with an identity -Both involve romance You get the point : 

Saint tail helps other peoples too, solves problems. but there's more romantic in it! though i have to say that Princess tutu is more artful to enjoy story. i love those both stories! its recommended by me! have fun watching :D 

I think this idea has been a bit overdone. KST is about a female teen who uses magic and illusions to steal items, returning them to their rightful owners. The only difference is that her best friend knows about what she does because she's a Catholic nun in training. 

both romance/comedy and have interesting plots involving "magic" 

both magical girl/ comedy/romance. they are both lots of fun and has serious moments/arcs too. 

Both series have thiefs that don't steal for their own good. 

Well Although different but very much alike. Magic... check Romance... check Transforming... check Well similiar 

They both had cute romance ,and they both had drama and action in both. But in some ways there are some differences but I think you'll like both.^.^