I'm not partial to either name really.
To me
Ivy has a poisonous feel to it. A temptress like Poison
Ivy on Batman(?). Though I do know a very sweet asian lady named
Ivy.
There could be bad jokes about her being as poison as ivy, or something about an ivy leauge. So old she's covered with
Ivy. *shrug* sorry.
Iris is kinda boring to me. I do know a very sweet older lady with red hair named
Iris so that reminds me somewhat of her. I like most types of
Iris flowers. How can rainbows be pretty and flowers not?
Iris...it just sounds funny.
Between the two I'd pick
Iris I guess. I don't know.