is there any difference in effect?

Scanning in this context implies understanding what is in the image. Hashing doesn't. A hash of a tree only tells apple "This is not a CSAM image", it doesn't say "there is a tree in this image."

ok i thought we were talking about "scanning for CSAM images". I dont understand why we derailed to different image identification techniques. The point is, whatever apple is doing on the phone could be done exactly in their icloud servers

I believe the difference is that on device they can do it prior to encrypting the image. On the cloud it is encrypted.