Palin doesn't scare the Democrats. Many are hoping she'll run. At worst, she'll split the conservative vote giving Romney the nomination and he'd be a president liberals could live with.
They're scared of Perry. They believe that not only is he electable, but he represents the worst type of right-wing, Christian cowboy that scares the wits out of them.
Don't believe me? Go peruse some DU threads on this issue.
I think Palin fear for them is on a whole new level, which if she runs will eclipse any fear they feel of Rick. I do think they fear Rick too, in a more conventional way.