Prime had much more work in terms of assets (geometry of the environment and 3D models and items) and textures, lighting compared to SM RPG.
It had also QoL improvements and new control options.
And I don't see what assets were kept from the original Prime ? The animations, sure. But graphically ?
I don't want to diminish the dev work for the SM RPG remake, but the art style is much more simplistic and less detailed than Prime, thus remaking the assets demands less effort. It's also a 1:1 remake in terms of artstyle, so you "just" translate the 2D assets into 3D.
Prime had to reivent all the assets to make them beautiful, remade, and keep the same original feeling.
SM RPG has made new 3D in engine cinematics, so there's also an addition here.
To me, those are really similar remakes, mostly on the graphic part. The new bit of content is so scarce it's not worth of mention honestly (a new boss and damaging side ennemies yay). And certainly not worth of calling it a true remake (Ă la RE2). I mean a lot of ports on GBA (FFIV for example) had much more new content, there were still ports.