I used to think the same thing, and there is some truth in that (like songs, whichever version you hear first you like more).

But I've read books after the fact many times, and seen movies second (of course) and if you are a type of person who gets absorbed into whatever you are indulging at that moment, it will vary, and sometimes you will appreciate both for what they are separately. I read Sense and Sensibility and About a Boy after watching the movies, and both have significant differences, but I felt in both, the director kept to the spirit of the book, and never changed the moral, or relationships. Sense and Sensibility is one of my favorite moveis of all time.