I just recently finished Shattered Dimensions and I played EoT first. Shattered Dimensions is a much better game overall due to slightly more varied gameplay, but the controls and gameplay for common Spider-Man characters are essentially the same between the two. Granted, you only have the 2 Spider-Man options in EoT unlike the 4 in Shattered Dimensions.
Both have a set of challenges you have to do through the levels, hidden spiders to collect, etc...
Outside of having Ultimate Spider-Man and Noir Spider-Man levels and a different boss at the end of each level, the games are pretty similar. As a completionist, Shattered Dimensions was much more difficult to complete.
With that said, $20 is way too much for this game as old as it is.