There was a recent discussion on a gear forum why people were ditching their brand new Sony A7 mirrorless cameras and were going back to full frame Nikon DSRLs like the D810. If you want to save time reading the rest of this post, the biggest thing affecting you in the mountains will be the very limited battery life
of mirrorless cameras. But there is far more that matters if you were to go out and buy one over the other for all your photographic needs.
Some of the points listed below do not apply in the mountains, but for me all this matters, as I also shoot things like motorsports and certainly can't afford a top of the line mirrorless body just to save a pound or two from my pack in the mountains. I own the Nex-6 mostly for video, as it does that pretty darn well for a $300 ebay purchase. I have taken 5 still photos with it over the last year, I think, only to test the lens adapter.
Take your pick of reasons why you may want to bring a D800 over a crop sensor A6000 or better mirrorless (most of the reasons below are comparing to a $1500 A7 II that needs a $300 adapter for my lenses):
- The image quality of the most recent Nikon bodies is superior to more or less anything out there, and that's just a fact (D800/e/810). Sony RAW files are lossy, which pixel peepers who need the max IQ will care about more than somebody who never prints or crops.
- The lenses available for Nikon DSLR cameras cover any need, as you can use any F-mount lens made over the last 60 years. Using adapters on mirrorless bodies is a pain and you lose much of the functionality of modern lenses (VR, AF, sometimes even metering).
- The weight advantage for mirrorless mostly applies with smaller sensors - full frame mirrorless is barely smaller and lighter even at the body level
- Batteries in mirrorless cameras have an absurdly short runtime - HUGE deal for me doing 3-4 week trips with few resupplies
- EVF viewfinders like the one in the A7 are useless in moderate light (forget poor light). D800 and especially D810 have some of the best viewfinders ever designed
- Mirrorless cameras have limitations when used for action photography, handling and viewfinder, focus tracking and prediction. They just don't work with moving subjects (wildlife maybe, but otherwise not relevant in the mountains)
- Good luck autofocusing on anything that isn't completely stationary using a mirrorless. These cameras do not have the processors and brains to track subjects in motion. They are an evolution of cameras designed to take family pictures with face recognition, not race cars or birds in flight.
- flash photography - HUGE difference if you do any studio work. Mirrorless is pretty brain dead when it comes to using a remote speedlight with TTL and other more advanced artificial light systems. Nikon's flash system is so good, I switched from Canon to Nikon back in the film days already, and that when I was actually shooting professionally and had a large investment in Canon.
Today, I only shoot for fun, but flash is a big deal when not pointing your camera at a big mountain. I agree that in the Sierra, I only use the built-in flash to fill in the occasional portrait.
Also to be noted is how quickly those high end Sony full frame bodies are depreciating (down to about $800 on ebay, which is flooded with these things). Sony life cycles are short, so they will keep dropping in value. I may pick one up once they go down to $500 if I can figure out how to keep it charged for a week between resupplies without carrying a pound or two of batteries.