Both are very similar.
Have the same synchro and the same character - qualities.
In these animes aim is to fight the evil ... whether with magic or a sword.
But the love is not too short and in both animes.
The anime also has what funny, the comedy is there, too.
Both Shana and the main female lead have similar characteristics! Both have flaming swords! Both have the same shounen, romance, fantasy action feel! DEFINITELY A RECOMMENDATION to watch while you wait for Shakugan no Shana 3 to come out!!! Also the main male lead is guranteed to become one of your favourite characters in anime! :D Pretty enjoyable to watch!
Both involve Season two of their counterparts, and how the lead male characters and lead female characters relationships have gone to the next level.
In ZnT they both realize their relationships that they love each other, although they stay far from it.
In SnS the lead female feels emotions unknown to her, thus she does not know how to react to them or what to do.
They both involve romances between inexperienced young children and the characters are both very similar. Nagasumi looks incredibly like Sakai Yuji and is as weak and uncertain of the circumstances as he is. Mawari Zenigata looks like Matake Ogata, both strong tomboys and Seto San vaguely resembles Shana, though personalities greatly differ.
I kept thinking of this series while watching Shana, a boy/girl pair fighting evil, some cute romance, love triangles and lots of action (though there is no magic in FMP) but if you like one, you'll love the other
Both give off a very similar vibe. Both have very intriguing plots. The art in both is similar especially during action scenes due to the many special effects used. Both have pretty much the same genres except To Aru Majutsu no Index has more comedy.