That's a good question. As far as I can tell it's mostly color scheme

But seriously, I'm not sure. I assumed it was just the new version that they'd be working on going forward. I don't know why they wouldn't just continue work on the existing version and avoid confusion, plus it's kind of dorky in this day and age to add the letter "X" to the end of something. But it's the keyboard I use on my phone and my tablet. It does a great job.

If you're a current Swiftkey user, I believe you get the upgrade to the X version for free...