A week later, I still haven't found anyone who could give me a good description of what each does and how they differ. So this afternoon I decided to make a Richard Schmidt color chart and see if I could find out on my own.
I mixed each with white, cad yellow lemon, trans-ox yellow & white, alizarin & white, cobalt & white and ultramarine blue & white. My conclusion: Veridian may be a tad cooler. It requires a bit more paint to tint the same amount of color. After that - duh. They seem pretty much interchangeable to me. According the Gamblin Colors, they are both lightfastness I. If you don't see much difference in the colors on the chart, that is because there isn't one!
So, I'll use Phtalo judiciously at half the cost of Veridian! If any of you have other observations, I'd appreciate hearing!