Will try to be brief but will not be easy.
I had a full pentax system and was an active member here many moons ago when i had lots of time on my hands. Had a baby son and time got the better of me, i was not using my gear other than the K7 with the 43 Limited for portraits of my boy. I was also tired of carting a DSLR around so bought a Fuji X10 for "life documentation". I loved it (still do), Pentax like colours, great sensor for it's size and soooooooooo brilliant to use as a tool. Meanwhile, after years of being agitated by Pentax's shoddy auto focus i finally grew tired of the 43mm missing it's mark, soooo much back or front focus, drove me crazy.
So, sold it all up and stripped it down to the X10 for weekends and the Fuji A-1 with 16-50mm kit lens and 60mm macro for portraits of the lad. Immediately hit a snag, without a big flash it was a nightmare and the 60mm AF was DEAD slow. A shame as the fuji, again, had utterly brilliant image quality and the photos looked like Pentax photos but you know, in bloody focus.
Sold the 60mm and kept the A-1 and kit lens as my weekend camera, deciding Fuji was a no-go for indoor portraits with uber expensive lenses and flashes. Second baby landed a few months back so decided to go DSLR again but keep it REAL simple, as in one camera and one lens. Settled on the Nikon D7100 with flash for an utterly ridiculous price, along with a Tamron 28-75 /2.8. Last month the first bioy started soccer so picked up the 70-300 VRII used for again, an utterly ridiculous price ($300 CDN).
So my impressions?
Fuji is so nice to use, very intuitive, wonderfully built and such great lenses. I MUCH prefer mirrorless than mirrored cameras but the system is hideously expensive and AF speed for moving subjects is not much better than pentax. Colour is so fantastic as well, those cyan biased blues and rich greens ala my Pentax limiteds. Alas, for indoor use with a fast lens and big flash, small cameras are awkward at best.
What about the Nikon? First the bad news, everything is arse backwards, feels totally alien and it is a very, very steep learning curve. Menu's are needlessly complicated and i still do not feel at home using the thing. A very capable device but dear me the combination of "everything is backwards' and the clumsy shooting modes still loses me.
My big issue though, is colour, it's better with the Nikkor than the Tamron but still, nothing looks right, ever. AUTO WB is utterly dreadful, i need to use custom settings in almost all lighting, the default is far too warm and far too green, very "muddy" for want of a better word. Also, to my shock, AUTO WB when using the flash is miles and miles off, i need to custom set it, with Pentax, AUTO was bang on. Super annoying.
However, in saying all that, sorry to report but the AF is so good it brings one to tears. In general it is spot on (I prefer to choose my AF point) but this last week was an eye opener. My boy playing soccer so i set the thing to AF-C, AUTO select, turned off the VR and went at it. Dear oh deary me, it tracked him like a god-damned missile. The keep rate was absurd, especially for those moments when the boy was running AT ME, it still tracked correctly. As a mechanical device, it is phenomenal, as is the 70-300 TBH.
So as usual, no one-size-fits-all but that's my 2c. Happy to answer any questions.