Noise is a "slippery slope", it is the area of sensor performance that has shown the greatest advancement in the past decade and for a given level of sensor technology, full frame tends to provide better performance. But the full frame versus APS-C is also very much a function of pixel density and a smaller number of pixels that are larger in area will perform better because they have more surface area. One of the reasons for the excellent low light noise performance of the 1DX series is its sensor density (20 megapixels) is constrained to a reasonable level which greatly improves its low light noise performance. I also have a 50 MP EOS 5DS R which can produce some gorgeous images but it cannot come close to the low light performance of my 1DX III with its more advanced and less dense sensor even though both are full frame.
Be careful when comparing sample photos showing sensor noise because scene composition has a lot to do with apparent sensor performance, if there is extreme contrast and the important areas are all brightly illuminated then apparent sensor performance will appear better than when using the same camera and lens with a scene where much of the important scene components are in the "gray" area between being brightly illuminated and so dark as to nearly disappear in the scene, those mid-level illumination areas are where noise issues that are difficult to remove in processing without significant loss of detail appear.
One of the big issues you will notice with noise is that it strongly impacts your ability to crop without very noticeable loss of image quality, when you can keep ISO below 400 you can crop severely and still retain good detail and image quality but at the higher end of the ISO range even using the full sensor area for the final image won't look so great and if you try a severe crop it will look truly terrible. When shooting sports, my EF 300 f2.8 lens is far easier to manage (lighter weight and less critical framing) compared to my EF 400 f2.8 but that 33% difference in focal length is significant when cropping in low light fast action scenarios.
Most cameras developed and marketed in the last few years have pretty decent noise performance because sensor design has advanced so rapidly. An EOS 1D Mark II was my first serious digital camera, I bought it in 2005. It topped out at ISO 3,200 but anything above 800 was really not good. My EOS 1DX III has a top ISO of 819,200 which isn't usable but I don't hesitate to shoot sports at ISO 12,800 with it and have gone higher when necessary.
Canon has an excellent AF system for their better DSLR bodies including the 90D, I don't think you will be at all disappointed with its performance. AF always occurs with the lens wide open regardless of the actual image capture setting so wider aperture lenses provide better AF performance, in low light it means the AF will achieve lock more quickly with less hunting and searching.
At some point I will buy a mirrorless body to play with, but Canon (nor anyone else) has a mirrorless body that does what I want. The AF of the Canon 1 series will capture focus more quickly than any of the mirrorless bodies when rapidly switching between two areas of interest which is a very common occurrence with sports, it is one of the advantages of having a dedicated separate AF sensor. I personally hate electronic viewfinders compared to the real time optical viewfinder which has zero lag or artifacts. And with the 1DX III, I get around 3,000 captures before the battery pack is below its half discharge point. Depending upon your usage, mirrorless may be the best choice but it isn't there yet for what I do while the mature DSLR system works extremely well for me. If I keep shooting long enough, I will transition to mirrorless when/if it overcomes some of the drawbacks that are important to me but at this point its limitations are not acceptable for my usage. But for your intended usage, a mirrorless could be the best choice. There is no single answer for everyone.
Be careful when seeing the marketing blurb for ISO range, just because a camera allows a very high ISO figure doesn't mean it is something you are likely to find usable. Sample photos below are my 1DX III at ISO 102,400 which might be usable in some situations and ISO 819,200 in which you can tell it is a cat but not much more 🙂 Really high ISO settings are a bragging right and not a terribly useful setting.
Rodger
