I like both, but prefer
Darla, and I don't really think
Darlene is useable. It's a television-only sort of name for me. My sole association with it is the daughter on
Roseanne, and I don't think
Darlene or
Darla ever made it to Britain, because I've never seen either name in real life.
Darla seems much less dated, probably because
Charlene etc are very 70's/80's whereas
Carla is pretty timeless.