The biggest improvment in Canon sensor tech recently is that the 5D4 has more detail in LOW ISO images, and you can push in detail from the shadows that all previous cameras would have recorded as noise. Oh, and if you shoot video, the 5D4 can record in log mode now... but you'll need to post process your video.

Having used a 1Ds2, I was amazed by how it handled highlights compared to the crop bodies (T1i, 7D, 7D2). I'm not sure such improved highlight handling is in a 5D series, or is 1D specific (I read an ancient article about how the 1D series did gain processing in analog, while other bodies did it digital, which could account for this, but I have no idea if a) it was even really true at the time, or b) if either half of that statement is still true today). This improvement is likely more important than pushing LOW ISO shadows, or log mode video to most shooters.

If you are in the USA, then LensRentals is an affordable way to give the two bodies a brief trial run and judge for yourself.