What is the difference between something like this http://sephora.com/browse/product.jhtml?id=P173625&categoryId=S15110&shouldPaginate=true and something like this: http://sephora.com/browse/product.jhtml?id=P137406&categoryId=S15110&shouldPaginate=true How will the sonic one benefit me more than just using a manual facial brush? Thank you.