I like them both.
Being a
Heidi, I am biased towards my own name. I have never felt it was stuffy, but rather people seem to think it fresh and cute. But, it still has that grounded feel that means it sounds like and adult name despite the 'i' ending.
Gretchen is one of those spunky names that you don't hear often. I happen to really like it. It's almost hipster on a child and chic on an adult. It has a lot more frumpiness than
Heidi does, but I think
Gretchen would be more likely to land on a kid's birth certificate today than
Heidi would.