I love Della its classic and sounds trendy, but isn't it. It's a favorite of mine. Willa is okay but not one of my favorites. It sounds 90's to me. Della could be a nickname for Adelaide, Adeline or Adele, which are all rising. I also think it's wonderful on its own. Willa could be a nickname for Wilhelmina. Both work fine on their own as well. Suggestions although I would use/love Della include:
Alma, Elka, Glinda, Lara, Lola, Lulu, Elsa, Gilda, Darla, Paula, Quilla, Velma, and Wilma. Also Matilda nn Tilda. Good Luck!
I like both, but prefer Della!