I get asked this a lot, and have strong feelings about it, so I thought I’d quickly put down a couple thoughts.
For portraiture you absolutely have to use lead white (Cremnitz, flake, d’argent etc…). Titanium white kills all the other colors, and lacks the beautiful transparency that almost mimics human flesh that lead has. Every great portrait was painted with lead.
For plein air painting, on the other hand, I think titanium is vastly superior. Keying a sky with lead takes hours, with titanium just minutes. Lead can still be useful for impastos below the horizon (especially if hand-ground), but titanium is really all you need outdoors.
I’d just like to add that anyone who disagrees with me on this is wrong.