I have do disagree with Vinvi, sorry. Although the two last ones have a very appealing use of contrast with the white and the sepia tone, I find that the two first ones are much more appealing... Why? The pictures taken is photographically appealing because of your use of rules of three. I like esp. the one on the RHS. The models pose and her positioning on the shot allows much of the green grass to create a very nice contrast with the complexion of the model's skin as well as the beautiful dress you designed. The green highlights the white of the model's dress which is very suggestive in terms of her purity and innocence. So really it depends on what sort of concept you have behind the series of images.
On the other hand, the sepia toned images, doesn't do the model, dress and the surrounding environment justice because you have limited the colours to be sepia toned and white (Although, I still quite like them). The sepia tone, of course creates an aged look on the photograph.
Although your sepia toned images are successful in creating an aged look, by looking at the models facial expression and body language, I find that the coloured images, does the model, the dress, and the surroundings, a lot of justice.
Hopefully, my opinion helped out with refining your concept and idea...
Goodluck.