These two books are similar in that they introduce new NPCs to the game. They are both a follow up to the NPC Codex released in early 2013, and act as a functional “Monster manuals” for NPCs, profiling the characters and giving them some background and tactics.
Of these two books, I would say ISNC is actually the less useful of the two over the long term – yes, it profiles a bunch of new NPCs, but we already HAVE a whole book full of hundreds and hundreds of NPCs (all of the CRB core classes from level 1 to 20, plus five builds for CRB prestige classes) that are free to be reskinned for all your adventuring needs. The place where ISNC shines is adding NPCs that are of noncore races and PC classes added after the core rulebook to the mix. So while it’s not AS useful in building adventures as the Rival Guide, it’s quite a worthwhile addition to any GM’s stock (as well as a useful way to suggest ways for a player to expand their usual horizons in character building).
The Rival Guide presents a group of NPC adventuring parties to act as rivals or allies to a PC party, and this is a useful one because player characters can sometimes grow sufficiently in power that it’s actually difficult to threaten them with ordinary monsters. In a case such as this, throwing a rival at them who is just as powerful but optimized in a different way can remind the PCs that they are not the gods of this setting and they can still be threatened.
Plus, nothing helps define a player character party like the opposition, and the rival guide presents some quite compelling opposition!
So, both books are worth buying. Both have their place in a Pathfinder campaign – although if you’re not running your campaign in Golarion they might take a bit of adjustment to get them ready to go for your own campaign world!