So many similarities...where to start! The basic, shared conflict in these is that the male heroes have spent many years attempting to return to a girl very precious to them that they lost due to tragic events. Once legends for their battle prowess before the latter happened, now their ability has greatly dulled over the years from going into hiding. In addition to the beginning plot, both of each novel’s second volume’s plot premise are identical as well. The chivalrous male protagonists are feared by many for their uniqueness: in Seirei he is the sole guy able to make spirit contracts and thought to be the legendary demon reincarnation, while in Hyoketsu Kyoukai he is the only existence to use the powers of the vile antagonistic creatures (a lengthy divine verse must be chanted for power releasing in both series). Finally, countless females have gathered in a single area to train and become “maidens” for a shot at their wish being granted or to be one of the few tasked with protecting the world. Despite the numerous similarities, the harem of tsunderes and perverted situations in Seirei might turn you away from reading if these features tend to limit your enjoyment.
TL;DR: you're going to have the same opinion of one that you had for the other.
The year is 2022, and gamers have lined up on launch day to connect to Sword Art Online, a hotly anticipated MMORPG that allows players to connect to an immersive, lush virtual reality world with special helmets called Nerve Gear. Kirito is eager to return to the place where he spent a good deal of time as a beta tester, and quickly becomes friends with Klein, a newbie player. But soon, Sword Art Online's 10,000 players discover that not only are they unable to logout, the only way they can return to their physical bodies is by beating the 100-level tower's final boss - that is, if their HP doesn't drop to zero first, for death in the game means death in the real world. Now, with no one else to turn to, Kirito and the other participants must survive the game as best they can – some forming guilds, others ruthlessly leaving their peers behind, and many tragically falling by the wayside or to the merciless monsters populating the lands...
The setting is a massive floating continent with a gigantic tower; SAO's has 100 floors and Hyouketsu's with over 200. The sword-wielding protagonists wear remarkably similar black cloaks that would make you think both Kirito and Sheltis were designed by the same person, and each fight to protect a girl dear to them, as well as thousands, using their unique, exceptional fighting ability. On another note, there's also comparable artificial lifeforms who are with the two protagonists constantly - and they're quite likeable.