Hm...I like both. Greta might be more modern, though.For me, I've always been in love with the spelling Grethe, but that would be difficult living in the US atleast. Looks like "Greeth".
I'd stick with Greta.I like Greta (as well as Gretchen, Griet, and Greet) but Gretel is so linked to the fairytale for me that it immediately brings to mind fat, rosy-cheeked German children.
The only problem I have with Griet is that I live in England, and it doesn't really look or sound very English! I'm not against different sounding names, but I think the pronounciation and spelling may be stumbled upon.