Custom image size vs CSS sizing

Question

I am developing a theme which will have 2 configuration options for the slideshow in the header; a full width version and a half width version. The theme is also responsive and so I am trying to optimize it as much as possible from a speed point of view. I am trying to decide which approach is best.

1. Use add_image_size() to create a new image size to use for when the user selects the half width configuration
2. Use the full width images and re-size them using CSS

The advantage the new image size is that the slider images would be smaller, this is positive since the theme is to be responsive, however it means that every image in the library now will have an extra image created, even though only a small few of these images will actually be used in the slider.

Which would be the best approach, or is there anything else to consider?