D610 is a nice camera, and it does indeed give you access to a wider range of lenses (from Nikon of course but also there are quite a few Sigma, Tokina and Tamron lenses that aren't in Pentax) and 3rd party accessories too (like flashes and flash triggers). If that is important to you.
But speaking as the owner of both a D610 and K-1, the K-1 has slightly better image quality, better high ISO (
DxOMark confirms), and the availability of a myriad of nice features that the D610 lacks, like A FLIPPY SCREEN, excellent cropability, GPS, focus peaking in LiveView, pixel-shift, and
WR too, of course. Plus it's a real solid build - the D610 feels like a toy in comparison.
D610 also sucks for low-light AF - it's AF is only rated down to -1 EV light level, which is the same as the K-x and K-50. So it compares poorly in that dept with the K-1. K-1 AF works down to VERY dim light levels of -3EV.
While I think the D610 is a great little camera for day-time events, if you really want to go Nikon FF, the D750 is probably the camera I would recommend. But if you manage to come across a K-1 for a good price, I would steer you that way. It's simply a great camera.