I think both are extremely important, and I would try to do whatever I could to ensure that I had done/have plans to do both before applying to dental school. If you can only do one or the other (not sure why this would be the case, but if it is), shadowing is probably the better option. It shows you have spent time in an actual dentist's office and know what it is like. You have that first hand experience and it shows that you know what your getting yourself into.
Again, if I were you I would try to figure out a way to do both, but if you can't, shadowing would be the way to go, in my opinion.