What they share
Both are flower names with strong vintage associations. Both are two-syllable with clear sounds. Both have been in English use for centuries. Both work for every age.
Pick Iris if…
Pick Iris if you want the mythological layer. Iris was the Greek messenger goddess who travelled on the rainbow, and the flower was named after her. The name carries both meanings at once.
Pick Violet if…
Pick Violet if you want the softer, more Victorian flower-name. Violet means the purple flower and carries gentle, literary associations (from Downton Abbey's Violet Crawley to Violet Beauregarde).
Iris is a touch more striking; Violet is a touch more sentimental. If you love flower names and want one that travels well in any register, either holds up.