These are differentessentially because they have a different chemical makeup.  This is akinto how a tomato and a pineapple are different, even though they are bothvarieties of fruit.  The molecular differences in particle shape oftitanium dioxide and zinc oxide play a significant role (and how they eachaffect light/UV rays) in determining their ability and “skill” at deflecting UVrays.  
       For example, if you were to shine a flashlight through a clear glass bottlefilled with olive oil, less light would be visible on the opposite side of thebottle than if you were to try the same experiment using a clear glass bottlefilled with water. Olive oil and water conduct light differently due to thesechemical differences. In the same principle, titanium dioxide and zinc oxideconduct light (UV rays) differently because of their moleculardifferences.   
      That aside, how these actives are formulated has a lot to do with their abilityto protect skin against UV rays.  It’s common to hear that titaniumdioxide isn’t as effective at deflecting UVA rays as zinc oxide, but whentitanium dioxide is formulated in a specific way (having to do with how theparticles of titanium dioxide are developed), they can equal the UVA abilitiesof zinc oxide. Hope that helps!