Phtalo Emerald vs Veridian - what's the difference?

I recently attended a workshop and found Veridian on the supply list.  My first teacher insisted that I use Veridian; since then it's always been Phtalo.  I (brilliantly) left my paint box home when going to this last workshop, so I spent an hour and some bucks at the local art shop.  When I came to the Veridian, I was surprised to find that it is considerably more expensive.  And NO ONE I asked could tell me the difference other than that "they are different chemical compositions."  I have lots of Phtalo at home, so I picked up a small tube of Veridian.

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!