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Welcome to FredMiranda!
I moved from film to digital about 11 years ago. My emphasis was sports, so my first digital body was the 1D. 8fps and RGB snowfield at 1600ISO - but that didn't stop me from using it! We're now roughly 5 generations ahead of where I started with the 1Dx (in the 1D series) not to mention the other D?? and ?D series.
As a general rule: Each new generation of body will have better high ISO capability. Meaning that the lower ISO ranges improve also.
Also, generally, each new series improves in AF capability and/or options/features to help you achieve more 'programmable' results for your shooting style.
Finally, with each new model, AF resolution usually goes up - which is usually the marketing point - and where many photographers feel they NEED GigaPixel sensors.
As you probably know from the film days, a camera is just a tool. How you understand how it works and use it in the manner that fits your need is paramount to the technical specifications of the tool.
For whatever camera you choose to get, there WILL be limitations. It's knowing how to get around or live with those limitations (i.e.: read the manual a few times) that will make you love or hate your investment.
That being said, it is also generally assumed that with each new generation of body, the tool is designed better and it will help/allow/assist you in getting more keepers if you are demanding of it: whether it be in the range of high ISO or more tuneable AF performance.
As I always tell new startups (though you're not one of them): get good glass first. That's going to make your photos better than any body will. And with good glass, it will last you through several generations of bodies.
Personally, my common shooting gear select is a Canon 1D MarkII-N with 200/1.8 lens. And I shoot it at medium (4Mpix) resolution. For my nichè, I shoot youth sporting events, where we take up to 200,000 images per day. We fill our frame with glass, the glass gives me enough light for decent ISO and I already have 22TB of storage filled up... just two years worth of images.
I'll be moving to the 1D4 and/or 1DX soon, mostly for serviceability, and eventually to newer 200/2 lens, but for our needs, I don't find myself needed to spend $6000 every 18 months on a new body. (multiply that times 3-4 as my wife and daughter shoot also and I commonly shoot a remote)
Recommendations? Again, for me, start with glass. If Nikon has better glass for your situation, get a Nikon body or vice versa.
I've read on these forums that generally, Canon is better suited for longer range stuff: mid to super telephotos and such, Nikon is better for wide to normal. Canon has it's emphasis on speed (fps), Nikon seems to have chosen the quality route (colors/low noise).
If you look back over the years, it's easy to see how they generally leap frog each other with each new release - but lately it seems to be more what you're looking for: speed vs quality. This is on the high end bodies. For all the mid range stuff - I think its a crap shoot and one would need to compare spex to find what each is capable of.
So depending on your needs, there are plenty of solutions out there - maybe some more specific to your nichè.