The GTX 690 has 4GB of VRAM total, meaning each GPU effectively has 2GB of VRAM to itself. This is soon becoming a limiting feature in modern games, despite the GTX 690 itself because even slightly faster than a GTX 980, providing the VRAM usage does not exceed 2GB.
Is it WORTH upgrading? That is really hard to say, you will essentially just be upgrading your VRAM, and removing any SLI issues you're having. I would say if you can SELL your GTX 690 for a price decent enough for you to justify "sidegrading" then that may be an option. The Titan X whilst being a very fast single card, is terrible in price/performance ratio.
It really all boils down to how annoyed are you with SLI issues from your GTX 690? Is it worth enough to pay the money simply to remove that issue? Raw performance wise a GTX 980 is on par.