I got into an interesting exchange with someone recently about this.  In reading the following sources (see links below), I'd been led to believe that zinc oxide provides broader UVA coverage than titanium dioxide (assume micronized particles, transparent formulation).  Specifically, I've come to understand is that zinc oxide protects better at higher wavelengths within the UVA spectrum (UVA1 versus UVA2).  But the person I was in the exchange with said I was mistaken, that titanium dioxide provides equally good UVA protection as long as the concentrations of it in the product are somewhat higher than you would use if formulating with zinc oxide.