Yes, all — ALL — other things being equal, larger sensors will capture more detail than smaller ones of the same design and pixel count.

On a purely practical level, you need to make tests to determine whether a given piece of gear, and/or a given *system*, will meet your needs.

For most of what I do, for instance, Micro 4/3 absolutely suits me best. I record lots of video with important, single-system, onboard audio, and lots of stills. The results are most likely to wind up on smartphones, tablets, computer screens, projection screens, TVs, and video monitors. The results may also be viewed as PDF files, or printed to letter-size documents. I rarely print larger than 20x16 inches.

So full frame dSLR? Not enough AV options. APS-C dSLR? Same. Other mirrorless? Sony could work well, but would mean carrying a heavier and bulkier kit that would yield an insignificant difference in the work I do. And I HATE Sony’s menus.

But... for LANDSCAPES, a full-frame or even a medium-format system would be much better than m4/3 or APS-C, especially if you make large prints (30x20 or 60x40 inches). Even though the “standard” viewing distance for any print is 1x to 1.5x its diagonal dimension, more pixels and more details allow closer inspection. Joe Public probably won’t notice, or care. But the format nazis at your local camera club probably will!

However, in real life, all things are never equal. As you point out, a sensor is part of a system and should be evaluated as part of a system for a selected purpose. An APS-C sensor matched with a focusing system matched with a lens to capture a shot at a distance in daylight might yield the best large print. While for landscapes, as you point out, a FF sensor (regardless of AF system since you can get arguably better results manually focusing) during the golden hour would yield a better result. My point being, trying to generalize about sensor size in isolation from the system and goal is sort of pointless.

Actually the real "theories and numbers" support exactly what he said! We don't see the real part often on forums such as this, where relatively most people are not all that aware and repeat whatever they have heard.

If you want to print large, or crop extensively, or shoot action in low light... spent money on a full frame body. If you want fine detail, can always work in good light, and never print larger than 16x20... spend money on a crop format.

Of course the nitty gritty details are subject to endless nonsense arguments claiming otherwise!

Up until about a month ago I had been using APS-C and M43 cameras side by side for a number of years. It is my experience that both formats produce excellent results and most of the time are easily comparable with full format. I owned 2 x Canon 70D, a Canon 7D mark II, a Nikon D500, 2 x Olympus EM1 and an Olympus EM1 mark II. As someone who almost exclusively shoots wildlife I found the crop factors for APS-C and M43 a decided advantage. I always used full frame lenses with my APS-C cameras and Pro lenses with my M43 cameras. I recently swapped my D500 and EM1.2 systems for a full frame Sony A7III and Sony lenses - I won't bore you with the reasons as to why I changed to FF. What I will say is, I do obtain better low light images, especially when using the Sony 100 - 400mm G Master. But when using my Sony 28 - 70mm and Sony 90mm f2.8 macro in good light, I really do not see any difference. To qualify my reply; I confess I only ever view my images via a 55" UHDTV. I feel that I have merely changed formats, I do not consider that I have "moved up" to FF, as some would say.