I pronounce them slightly differently. GWEN-ith and GWIN-ith. But I think of
Gwenith as a masculine name (because of the meaning, and I swear I read once that it is/has been used that way too) and
Gwyneth as feminine. I don't really like
Gwyneth. It seems too airy to me. But I love
Gwenith (on a boy). :)