I have not used either lesson plan for Frankenstein, but I have used both resources in teaching other novels. Generally, I prefer the Prestwick House materials, especially the Response Journal. I like also that Prestwick provides study guides for both the student and the teacher. I can just print them and hand them out. With the stuff from Teacher's Pet, I have to do a little more work in putting the materials together.
I agree that you must adapt either one you choose to fit your students. I think both are valid and helpful study guides and teacher's guides. What I usually do is take some of the questions from the study guides and combine them with my own to make tests, quizzes, short-answer essay questions, etc.
Either one has excellent activities, but I agree with Tim in #2--get a feel for what your students need and talk about the book itself. Some activities are just designed to take up time, and honors kids will smell that a mile away.
I don't think it makes much difference because they are both overwhelming :) I would think there are some good things in each that you might want to use ... I have taken questions from each to make up an "objective" objective reading quiz. There are also good essay questions that you can pick and choose from. A lot depends on where your students are ... to me, it's always about teaching them and not the book.
I'd go with the Prestwick House plans--I used the plans for "Deathwatch" for gifted 8th graders. They give a good solid basis and some sound ideas to extend and differentiate with. They also were error free.