I have owned a Nikon D700 and currently own an XT3 and love landscape. I shoot handheld and with a tripod. I was a lifelong Nikon shooter and I consider the D700 a fairly special camera. There was something beautiful about the files that I would get from the camera. I used to have a Nikon D200 and was always a bit disappointed with the images out of camera with regards to dynamic range. The D700 caused me not to abandon digital and go back to film. There was a lot of data I could get out of the D700 files if I played with them a bit in Adobe. It will always be one of my favorites. Never tried the D750. Rented a D800 and found it didn't set off the same bells and whistles for me for some reason. Switched to Fujifilm in 2012 primarily because I was intrigued having a camera with no mirror box (less camera shake) and was lighter weight (hiking). Fujifilm and Nikon had worked together or supported each other technologically in the past. I love Fujifilm's color science from the film days.
I have had the X-T3 with battery grip for a very short time and have used it a few times. I like the way it feels and handles so far. I still need to do a big trip with it but what I see so far from the files I do like. I don't do a ton of post and I like to tweak things in camera. I can tell you my past Fujifilm ownership has included ownership of the X100(gone), X-T1 (gifted to family recently after several years of use), X20 (gone) , XPro1 and XPro2(keepers) along with the XE3 (selling). I am very happy with the image quality from the Fuji products. When I compare my D200 shots from Yosemite and my XT1 shots from Yosemite the Fuji shots were fantastic. Also, I have found I can recover a lot of info from Fuji files if need be particularly in the shadows.
You could always rent the camera and play with both to help your decision.
Lenses: Many folks swear by the Samyang 12mm. I own the Zeiss Touit 12mm which I picked up for sale in a bundle for $420 USD new. I found the 18mm equiv focal length useful in Yosemite and other landscape destinations. It sells for more than that still today. However my favorite focal length is 24mm equivalent so the Fujifilm 16mm f1.4 stays bolted to one of my X cameras these days. I used a 20mm Nikon prime back in the day but never felt like getting the Fujifilm 14mm. Some folks love the 14. I have a 9mm ultra wide (13.5 mm equivalent) and a 7.5mm fisheye (11mm or so equivalent ) that I am trying out on the XT3 this weekend.I've never been interested in trying the Fuji 10-24 zoom.
Hope this helps.