If you are not part of the wedding party, I would consider dress for the weather.
I would find out for sure where the wedding is being held and go from there. I agree, if you aren't part of the wedding party there is no reason to buy your dress at a bridal shop, unless that is your preference. I don't know that I'd go with the LBD, I'm more likely to go with a color that looks good on me, in a style that is comfortable, flattering and appropriate for the occasion and setting.
LBD = Little Black Dress