Toaru Majutsu no Index SS

Warning: These side-stories (SS), shouldn't be read without having read the main series.
SS1 should be seen only after you read vol13, whereas SS2's after vol16.

Overall these SS's where a nice surprise, the main series suffers a bit at some parts with dragged out sequences of the plot (mostly on the magic side), something not present here due to the SS narrative structure of tackling more individual short stories that end up connecting which each other and can even complement some aspects of the main series's plot.

I found SS1 more focused and interesting than SS2, 1st has way less chapters and bigger and more stories focused, with the highlight being the foundation of GROUP, which plays a huge part on vol15 and so forward.

SS2 had too many individual chapters, some chapters interconnecting over a larger plot of the existence of gemstones and different factions fighting for it, alot of debuts for cast that then makes major appearences later on New Testament.
There were some highs, but also more than enough random and boring mini-plots (like the ninja's) that I couldnt care less about.

In the end, if you are a fan of the series, you should read both, just keep in mind SS2 will be slower to digest, and that might bring down the enjoyment with these SS's in the end.

Everything said, still had lots of fun reading, I would still give this a 9, more in detail, a 8.6 would suit for both of these (SS1 was at least a 9.0 for me, but had SS2 bring down the sccore).