I did not check the opacity of TiO2 versus BaSO4, my guess is that TiO2 is more opaque considering its use in paints and plastics.
As written before, TiO2 is absorbing UV light and Barite not, so in combination with OBAs TiO2 is not optimal. Of course there is then that choice to put OBA in the top layer, in or on the inkjet receptor layer. Worst place to battle fading but it will intercept the UV light before it is absorbed by TiO2. If you check some papers with identical weights and with TiO2, one natural, the other bright white, then I tend to think that they only add OBA on top of the natural quality to get the bright variety. Backsides almost identical but the OBA at the front will influence that reading a bit too.
As a side note; I see OBA use mentioned in some cases to enhance the strength of colors including the whites, with lamination foils for example.
Met vriendelijke groet, Ernsthttp://www.pigment-print.com/spectralplots/spectrumviz_1.htm
470+ inkjet paper white spectral plots, November 2012.