Both of them are WAY too dated for my liking. I’d probably pick
Velma if you forced me to choose one, just because it sounds less like a grandmother and you could nickname her Vel.
Wilma is just sorta weak sounding, while
Velma at least sounds kind of sassy.
Velma Mae Velma JoVelma Rose Velma Cassidy Velma Britney Velma Sophia Velma Isabella Velma Madeline