TAJ
Darkness cannot drive out darkness; only light can do that. Hate cannot drive out hate; only love can do that.
Oh god, is it like that? Horrible.
Not all of that describes both.
Do you need to keep the old games for both? Yes, but for different reasons.
In Infinity all story content is locked behind playsets and those playsets only work with the version of Infinity they were released for.
In Skylanders 1-2 hours of side story is locked behind playsets for each game and the bulk is playable without playsets. The playsets only work with the game they were released for.
Characters in Infinity can only be used in story mode with one playset. A lot can't be used for story at all.
Characters in Skylanders work in all levels released later. Characters from the first game still work. But newer characters aren't compatible with previous games.