Well, here Gréta is pron. GRAY-ta, so that's how I would say
Greta. Gréta is the name of the daughter of a distant cousin, and I don't really like the little girl, so I'm biased. I don't find it feminine enough.
Gretta is a bit prettier, but I like
Gretchen even more.
On the other hand,
Greta and
Gretta are quite underused, so it's definitely a plus. They also go well with
Jean IMHO (if you plan to use it for a daughter).
And all the roads that lead you there were winding
And all the lights that light the way are blinding
There are many things that I would like to say to you
But I don't know how(Oasis: Wonderwall)