On the surface, these characters could not be further apart. On the one hand, you've got Eren Jaeger from Attack On Titan, an obnoxiously (although rightfully) intense kid with some severe anger management issues, who fought for humanity's survival and eventually became the figurehead for the Jaegerists.
Then, on the other hand, Kirito (real name Kazuto Kirigaya) is an often goofy yet also serious youngster whose time in virtual MMO's turned him into an expert swordsman, lost between the real world and the digital one.
The only commonalities the pair seem to share are their English voice actor (Bryce Pappenbrook), the fact that they look the same - at least when Eren was younger - and that they're both the protagonists of epic franchises that have become two of the most popular contemporary anime.
Despite their only vague similarities, the duo shares one more thing which can be tough to distinguish, and that's their dialogue. Filled with thoughtful and passionate speech, Eren/Kazuto have some great lines in their respective shows and figuring out which one said it will be challenging. Good luck!