I like the backless dress very much but not for the wedding. My very first thought when I saw it as a pretty relaxed mother of the bride circa a few years ago was that it was a little bit of a show stopper for an event that already has a designated leading lady. Especially at a smaller wedding.
Not that anyone would ever dare to say that out loud.
With your red hair and fresh complexion the Eileen Fisher is probably very pretty on you for day or casual but so far the clear winner for a wedding in Hawaii (are those hibiscus on the dress?) is the Equipment dress. I'm thinking the wedding is probably not at high noon so I don't see why silk sleeves are a problem, even on the beach?