I've had the camera I have for a few years now (the Nikon d40x) but have been thinking about getting a new camera as the Nikon d40x doesn't have great low light performance. I've been thinking about getting the Sony Alpha a7r II but was wondering if that would be a good idea as I wouldn't be able to use my old lenses (I have three zoom lenses). Should I get the Sony Alpha a7r II or get another Nikon DSLR?
The one hesitation you mention is whether you can use your existing lenses, so I'm going to answer from that perspective. Ultimately the choice of whether or not something is better for you is a very personal decision that only you can answer — but this part is more straightforward.
Are your three Nikon zoom lenses high end, expensive glass? Since the D40x is an entry-level mass market camera, I assume that the lenses you have are too. In that case, I wouldn't worry about them at all — it'd be silly to spend $3000 on a camera body and then handicap it that way anyway.
But, on the other hand, if you saved money on the body and have high-end lenses like the Nikkor 24-70mm and 70-200mm f/2.8 at over $2000 each, you might want to stay in the system to match. (But, on yet a third hand, those lenses have a high resale value, so trading them in wouldn't be out of the question if you really want to switch.)
So, bottom line, unless your lenses cost significantly more than what you're considering spending on a camera body, don't bother factoring them in (and if they did cost that much, it's still only a factor, not the decision point).
I am in roughly the same boat (more lenses though). I think that Mirrorless is the way of the future for the following reasons.
- The very high sensitivity of sensors will allow (if not already) the EVF to give you more information in low light than a regular viewfinder.
- You don't have to take your eyes away from the EVF to see a preview of the captured photo, thus saving you from that moment where you realize that the last 10 shots are completely dark because you forgot M mode from last time.
- Lens stabilization no longer has advantage over sensor stabilization as you can see the stabilized image through the EVF, meaning 2 things:
- cheaper and smaller lenses
- stabilization even with prime lenses that don't usually come in VR/IS
- Less (audible) noise, less shake due to mirror movement
- Smaller camera
- Little things, like manual focus assist, histogram on evf etc.
However, the reviewer at fstoppers (https://fstoppers.com/originals/fstoppers-reviews-canon-5dsr-sony-a7rii-and-nikon-d810-89896) while he does acknowledge that mirrorless is the future, states that the A7RII is just before the crossover point. Given that the hardest part for an experienced photographer is to get over the tradition I appreciate the fact that they don't dismiss the A7 as a toy but rather make their case.
So for me the scales are level if it was about someone who would buy his first professional camera. However given that you have some lenses the scale tips lightly (or heavily, depending on how good glass you have) towards the Nikon.
There's always a third option: wait. The third iteration of the A7R will probably be clearly over the crossover point with the DSLR's and it will make the choice all the more easier.
I had a Nikon D40 and I switched to D7100 because I could keep all the lenses (total investment: 1000 Euro).
From my point of view (non professional) I think that was fine, I would not be able to justify for pleasure throwing away that much money to buy new lenses.
However, if I had to start from scratch, I would buy Sony Alpha and go mirrorless. The newer sensors are sharp, and phase focusing on sensor itself ("hybrid mode") brings way better results, since no microadjustments are needed. And you can follow subjects very well.
Battery is not an issue, you can have TWO spare batteries plus A7 for the same total weight of a similar dSLR, since mirrorless is lighter. And video advantages, and so on. Moreover, Nikon has very deep focal plane: you cannot convert other lenses to Nikon without additional optics. But the opposite is possible (manual lenses).
If you don't have many lenses, go Sony.