I shoot with Canon... their cameras are "comfortable" to me and the Canon system has most of the things I need.
But I know other folks who are just as happy with Nikon... or Olympus, or Sony, or Pentax, etc.
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....I purchased a D-70 years ago shot well with it, considering upgrade to a D-750...
If you have Nikon-fit lenses or accessories, you might be best sticking with Nikon.
D750 is a "full frame" camera, while your D70 was a "crop sensor, APS-C"...
Just be aware that full frame format isn't necessarily "an upgrade". Both formats have their pros and cons. But even the most entry level models of modern APS-C DSLRs are light years improved over your 6MP D70 from around 2004.
Full frame cameras (FX in Nikonese) are for all practical purposes limited to full frame-capable (FX) lenses. Yes, you can fit a crop-design lens (DX) to an FX camera and it will work... but that's sort of like buying a Ferrari and replacing it's engine with a 4-cylinder out of a Yugo... sure, the car might still go from A to B, just nowhere near as well. In contrast, a DX camera can use both DX and FX lenses equally well. On the whole, in addition to the FX camera itself usually costing more and possibly being larger and heavier, FX lenses are also generally bigger, heavier and more expensive. In fact, if you do a lot of telephoto work, a DX camera has some distinct advantages. The FX camera has advantages if you use wide angles a lot. FX cameras may be preferable for low light shooting (lower image noise), but a 2 or 3 year old 24MP D750 might now be rivaled pretty closely by a one or two generation newer DX model such as the 24MP D7200.
All that said about the cameras... they are actually more equal than they are different, even putting aside formats. It's actually the lenses used upon the camera that make more difference in your images. A lot of folks over-spend on the camera and under-spend on the lenses. I'd rather have a cheaper, more entry-level camera fitted with a higher quality lens, than the other way around.
Something you need to be aware of with Nikon is that their more entry-level models (currently D3000 and D5000-series) require AF-S lenses to be able to autofocus. AF-D and similar lenses that don't have a focusing motor built into the lens itself, that rely upon an AF drive motor built into the camera instead, can't autofocus on those cameras. The higher-end Nikon cameras (D7000-series, D300, D500, D4, D5) have the motor in the camera, so are able to focus both AF-S and AF-D lenses. This isn't a huge limitation, since a lot of Nikkor lenses have been converted to AF-S drive over the past 5 or more years. But if you are looking for something specific, you should check that the lens will do what you want on the camera you're considering.
Nikon still uses the F-mount bayonet that they introduced back in the 1960s. Modern Nikon cameras and lenses use a sort of hybrid form of that mount, updated with electronic connectivity. Many vintage manual focus Nikkors are usable on the modern Nikon cameras, too, giving potentially millions of quality lenses to choose among in the used market. With some cameras there can be "compatibility" issues with metering systems and such, though... especially with the more entry-level modern DSLRs. And the Nikon mount design doesn't allow for many other camera manufacturers' lenses to be adapted for use on Nikon cameras.
Canon went another direction, completely switching over to an electronic lens mount in a new bayonet that was introduced in the late 1980s. Canon's earlier FD/FL lenses are generally not usable on modern Canon EOS/EF mount cameras. A lot of people were really upset that the old Canon mount was orphaned at that time. However, the modern Canon mount is quite versatile and it's possible to adapt most vintage Nikon F, Pentax M42 and PK mount, Olympus OM, Leica R and other vintage mount lenses to be used on modern Canon cameras. (Note: Sony's mount is similarly versatile... while Oly and Pentax are more limited to only their own vintage lenses, like Nikon.) So on the used market there are not only the 125 million Canon EF/EF-S lenses made since about 1990... as well as millions of lenses made to fit Canon EF/EOS, there also are many millions of vintage manual focus lenses from a number of other manufacturers that can be adapted, too... including most vintage Nikkors!
But, basically, all the modern DSLRs are quite good and very capable.... even the least expensive, most entry-level current DSLR from any manufacturer will run circles around your old D70.