I had the Sea&Sea YS D1/2 before getting the Retra Pro strobes. They are superior to the Sea&Sea in almost every way: the quality of the light is a lot better. You are getting a more even lighting, less hotspots, better color temperature (affecting the white balance and the tone of the blue of the water), a more even spread of the light. Also they are made of metal and just feel more solid and better made. The Sea&Sea have a reputation of have not the best reliability, the Retras did not fail on me a single time. Also you have a lot of accessories that are easy to mount/dismount and giving you more options.
Recently I switched to the Seacam Seaflash 150D strobes which are just a little bit better in the quality of light than the Retras in my opinion. So you might want to consider those as well.
I don't know the Sea&Sea YS-D3, but for me the Retras were a definite upgrade to the YS1/2, well worth the money!