I recently made a similar move in reverse. I switched from a Sony NEX-7 with the Sony 10-18mm and the Sony 16-70mm lenses to a Nikon D810 with the Nikon 14-24mm lense, and my two cents is to definitely stick with the D800. The image quality on the Sony was great, but the D810 is clearly superior especially in terms of ISO and dynamic range.
Dynamic Range. This is the biggest difference between the two. I was pretty happy with the Sony's dynamic range, but I was blown away with the D810's range. The Nikon's raw files appear to have an extra stop of dynamic range in each direction. This is extremely useful as I now never have to bracket using more than two exposures (one exposure for the foreground and one exposure for the sky) even when shooting directly at the sun. You will definitely have to bracket more when using the Sony.
ISO. I was never really comfortable with the Sony's image quality when shooting anything over ISO 400. Anything over ISO 400 would result in way too much noise. With the D810 I can shoot ISO 1600 no problem, and even at ISO 3200 with acceptable noise. This difference in ISO is huge if you like to shoot at night or at twilight, or if you are shooting moving subjects like wildflowers on a windy day. The Sony's ISO was so bad that I never even tried to shoot at night with it.
As for, mexapixels. If you only prepare your pictures for posting online or making 4 x 6's, then you will not really notice the difference in MPs. But, if you plan on making some larger prints, e.g., 20 x 30's, then you will notice the extra megapixels.
These differences are enough for me that I will always take D810 with me for backpacking trips and leave the NEX-7 at home. Now, the differences are not massive, and I was very happy with the majority of the shots I got with the NEX-7. But, Fishmonger said it best. You don't want to get home and feel like you missed some shots - even if its just one or two - because you did not take the best gear with you. (P.S. That is the same reason why I would not go with a lighter tripod. I have tried using gorilla tripods and lightweight travel tripods, and I have found that I have missed shots because of it. The lightweight tripods are not sturdy enough when shooting long exposures or when it's windy. The gorilla tripods are sturdy, but you are limited to ground level shots. Now, I just hike with the same tripod I normally shoot with.)
As for weight, yes the Sony is lighter (the NEX-7 with the 10-18mm is less than 1.5 pounds while the Nikon with the 14-24mm is almost 5), but you need to remember a camera and a lense are only part of your total camera gear package: camera, lenses, tripod, filters, carrying case, memory cards, batteries (which you will need more of with a Sony mirrorless due to the Electronic ViewFinder). Everyone's kit is different, but for me, once I factored all these things in, the difference between the cameras ended up being the difference between carrying 10 pounds of camera gear as opposed to 7 pounds of camera gear. That 3 pound difference in gear can be made up elsewhere (lighter shelter, lighter backpack, lighter sleeping bag - definitely lose the camp chair and just sit on your bear canister). By going lightweight everywhere else besides my camera gear, I can head out with the 10 pounds of camera gear, a week's worth of food (1.5 pounds per day), and a liter of water and still have a total pack weight of just under 35 pounds.