Throwing the different MP sensors into the mix makes things really complicated.
I guess one way to look at it is that if you are happy with what you get from a 24MP FF system, there's every likelihood that you'll be very happy with what you get from a 40MP APS-C system. In the end, both can produce very good quality image files, though their pluses and minuses are both small and a bit different.
"More megapixels" can have a range of effects on your image file. When using excellent lenses and excellent technique, there's a chance for some additional overall image detail resolution. If you were asking the question about two cameras using the same format, the 40MP sensor would have that potential, virtually without question _ though the actual impact of the difference on your photography would be open to some discussion.
However, given lenses of equal resolving power (as measured in line-pairs/picture-height or similar) a camera with a smaller sensor has lower system resolution. To put it simply, there are "fewer millimeters to hold line pairs," so the system cannot resolve the same number of line pairs across the full image with the smaller sensor. But... do either of them (the FF or the APS-C) cross the resolution threshold in a meaningful way? Hard to know!
There are other potential pluses from a higher MP system on any format in addition to detail resolution. For example, any noise will have a smaller "grain size" on the higher MP system, and it is possible that a higher MP system may have some advantage in rendering smooth transitions and gradients.
I think there are a few ways for you to resolve your question.
1. As you note above, borrow or rent the 40MP APS-C system with good lenses and try it for yourself. (Fujifilm is putting on some "demo" sessions here and there. I just missed one in my area yesterday. I've been to them in the past, and was able to put my own memory card in a camera and make some test shots.)
2. Decide that the pluses of the smaller system are worth enough to you that given the size at which you typically reproduce your images, no difference is likely to be important. (In my experience, I'm completely confident about the quality of 20" x 30" prints from APS-C.)
3. Do a bunch of math to try to quantify the difference, and then try to extrapolate that to real world conditions. Good luck! ;-)
Dan
Nov 17, 2022 at 11:39 AM
Previous versions of gdanmitchell's message #16096518 « APS-C 40 mp vs. FF 24 mp »
