(Hi there, I'm Clay, the creator of the picture frame mentioned here.)
You're correct that the same panel is in the Dell display, but I'm pretty sure it's made by LG, not Samsung.
I purchased one of the Dell displays for testing purposes, and unfortunately, for reasons I don't fully understand, the color on the Dell display is quite a bit worse, with non-uniform color over the display from left to right, and also variable gamma at different brightness levels. The iMac also supports finer-grained control of brightness, which you need for luminance matching. All of this makes the iMac and its display a better choice for this application.
You're correct that the same panel is in the Dell display, but I'm pretty sure it's made by LG, not Samsung.
I purchased one of the Dell displays for testing purposes, and unfortunately, for reasons I don't fully understand, the color on the Dell display is quite a bit worse, with non-uniform color over the display from left to right, and also variable gamma at different brightness levels. The iMac also supports finer-grained control of brightness, which you need for luminance matching. All of this makes the iMac and its display a better choice for this application.