My 2c worth.
I've used both the D610 and the K-3. Both are nice, good quality cameras. You won't really go wrong with either, to be honest. Never touched the Xt1.
I'd be comfortable shooting either the D610 or the K-3 for musicians and bands. Perhaps more the K-3, because the focus is more confident in low-light than the D610 (the K-3 will focus down to -3 EV lighting, which is next to pitch black, whilst the D610 will only focus down to -1 EV, which is pretty average). White balance performance of both is great, viewfinders are equally good, live view works well for both, having a larger sensor means the D610 has the edge in low-light image quality, (but if you keep the ISO of either the D610 or the K-3 below ISO 3200 you would be hard pressed to tell their images apart). Shooting speed (frames per second) on the K-3 is also about 30% faster than the D610, and the K-3 has a bigger buffer than the D610. D610 has better video specs than K-3, but I've never used video on either, so can't really comment.
For outdoors use, and maybe bands in rough venues
, K-3 is the clear way to go. It's miles tougher, more weatherproof etc than the D610. The D610 feels like a plastic toy compared to the K-3 sometimes.
Nikon has some nice lenses, so does Pentax, so you have lots of great options either way. The lenses to choose will depend on the shooting purpose, but if money was not an issue... for Pentax go with the FA 31, DA 55, FA 77, DFA 100 Macro, DA*16-50, DA*50-135 or DA*60-250, DA*300
Last edited by rawr; 11-21-2014 at 12:56 AM.