The difference was profound in two categories: In ANH, Vader was already what Kylo Ren was not: a great, effective villain, so when the Emperor was introduced in ESB, the entire Star Wars world and its mystique grew exponentially by having an already legendary screen and in-universe villain in Vader in a genuinely submissive role to this grim, near-snarling projection. That's the opposite of Snoke, who was just a "we need a Sidious in this series" stand-in. He had no presence, no magnetism as an evil figure--just another painfully CG "bad guy" that meant noting, therefore his relationship to Kylo Ren (and anyone else) meant noting. The glaring contrast to Palaptine is that no matter how powerful and malevolent Vader seemed to be (especially after all of the damage he inflicted on the protagonists in ESB), there was this one figure--a true corrupt opposite of Yoda--generating fear and respect from Vader.
By the time of ROTJ, Palpatine was the most anticipated character after Luke, and long before his actual confrontation with Luke, he was a larger than life presence and influence on all plot points in the film. This was a highly successful character, hence the reason audiences were on the edge of the seat as he was electrocuting Luke, and actually on their feet cheering (at least on premiere night at the Egyptian Theatre back in 1983) when Vader hurled him down the shaft. In other words, the audience was emotionally invested in the Emperor enough to be absolutely overjoyed to see him meet his end.
That is the reason fans looked forward to seeing his early years when the prequels were announced.
Snoke was never developed, coughed up some sound-alike "Sith-isms" and was not the overshadowing influence on the lives of the heroes in the Palpatine was. So, you did not get much from that, but innumerable moviegoers, series fans, et al., thought his role--and ultimately death (in the OT) was one of its most important, memorable events.
At least Fett's original intent--to be a sinister bounty hunter--played a significant role in his debut movie. Phasma was the product of anything other than creating a character with impact on events. She made a couple of snarky comments about resident fool/clown Finn, stomped around, and that was it. There was no disappointment in Phasma's death, because she--like Snoke--meant nothing.