Definitely not both. Other than that I'd have to say it's a personal preference. I'd use css if I had many images the same size to reduce code.
.my_images img {width: 20px; height:20px}
In the long term CSS may win out due to HTML attribute deprecation and more likely due to the growth of vector image formats like SVG where it can actually make sense to scale images using non-pixel based units like % or em.
