Well with all due respect, the makeup is visually different first of all. In ROTJ his skin was sagging so as to make him look like a 200 year old sorceror; there were those weird deformations on his forehead but that was I think just to show that he was also somewhat deformed by his power. But more or less he fit into the traditional look of the "evil ancient sorceror." In ROTS its very different--similarities too, as one would expect considering its the same character and visual consistency is a given, but here he is given a look that is more monstrous, more horror-oriented, more resemblant of a Lon Chaney character or something. Also, perhaps because his appearance was now ambiguously linked to being deformed through an assault, the deformities were more exageratted.

Secondly, the aesthetic quality of the appliances are very different. In ROTJ it really just looked like flesh that had withered away, whereas in ROTS it has a plastic, sculpted look which highlights the depiction of the face not as an "ancient sorceror" whose appearance was due to the ravages of time and power but as a deliberate construction so as to appear monstrous.

I think the way McDiarmid played the role also contributed; in ROTJ he is quieter and somewhat labored, whereas in ROTS he is very expressive and energetic, especially in the end battle where the appearance really looks the most distant from ROTJ. The expressions of the artist under the makeup adds to the way the makeup looks. Thats why the scenes where he played things small and simple resembled the OT character more (ie some of the knighting scene, the scene where he reconstructs Darth Vader).