I have both a d810 and an A7r. What others have said is
true, my A7r does not hold up when it comes to AF and
power consumption. Since I do a lot of landscapes and
mainly use MF it is not a huge issue for me. The power
consumption is a legitimate concern, but I carry a large
number of spare batteries with me when I use the A7r.
Ad far as IQ, the quality from mirrorless is just as good.