Shakugan no Shana Specials, Toaru Majutsu no Index-tan Recommendations
Same studio, references to Shana, same style, etcetera.
Replace Shana-tan with Index-tan and you probably won't notice any difference...
Infinitely similar because they both have same publisher and studio. In fact, Index-tan even says so at some point in the episode.
Shana and Index tan are very much alike.
It's usually best to watch the main series first, you'll enjoy the humor more if you do.