What are pros and cons of using par rates v. zero rates for principal component analysis of daily history of a yield curve?

The seminal paper Robert Litterman, José Scheinkman (1991) Common Factors Affecting Bond Returns. Journal of Fixed Income, 1, 54-61. https://doi.org/10.3905/jfi.1991.692347 used the history of daily changes in observable par rates. Some people prefer to bootstrap & convert to zero/spot rates. What are the advantages/disadvantages of each approach, abd are there are any choices that one should consider?