Joel,
Since I don't think you are planning on super high RPMs and you are already using the steel retainers (which I think are great), I would go with the steel locks.

While I have never had a Ti retainer fail I have had them start to gain installed height as the valve sunk into the retainer. The last engine I had that had Ti retainers did not have many runs on it when I went to the Comp steel retainers so perhaps the newer Ti stuff is better.