The question is a bit of a trick, because the answer doesn't depend on fashion but on two things: the wedding dress code and the silhouette of the person wearing it. But if a clear recommendation has to be given, the midi wedding guest dress wins by a landslide in most current contexts.
Floor-length is the most ceremonial option and remains the right choice for very formal weddings or those with strict protocol. But the midi (which falls between the knee and the ankle) visually elongates the leg, is more comfortable for hours, and offers a versatility that floor-length doesn't always allow.
The Nadine Dress is the best argument for this style: an embroidered top with passementerie, a tulle skirt, and short sleeves—a design that has the same spectacular impact as any spectacular floor-length wedding guest dress without the stiffness of a floor-length skirt.
For those looking for something more understated and with more versatility for future wear, the grey Dalma Dress (three-quarter sleeves, spot print on grey) or the Blas Printed Dress (flowing, V-neck, batwing sleeves) are two midi options that can be worn to a wedding or a dinner without needing a change.
The practical rule: if the wedding is in the morning or afternoon with natural light, the midi is the best choice. If it's at night with a high level of formality, floor-length is the way to go.