They are about equal in color potential. Color aside, they have big differences in growth habits that have to be considered.

The Pistache is a much larger tree ... twice as big at maturity. Can be unattractive when young, but improves with age. Dense shade.

The tallow tree is prone to suckering and has to be pruned to shape a tree. Light to medium shade.

If you can, select the plants in fall ... color can vary from spectacular to ordinary within a group of trees. The color is stable from year to year, so a spectacular young tree is more likely to priduce a good show.