Generally, as per the above posts.
Certainly a beauty dish is no kind of panacea, they are designed to create the right shadows in the right places on the right face - and the results can pretty awful if they are used wrongly, or on the wrong faces. I view them basically as a half way house between a softbox and a fresnel spot, with the fresnel spot suitable for a young model who has a perfect skin, perfect bone structure and perfect makeup, and the softbox suitable for her mother...
Differences between white and silver and plain, and I wrote an article about it -which now seems to have disappeared from the website, I'll try to find the link. But basically, the white is less harsh, less dramatic and more suited to models who aren't quite perfect. They still require a high standard of skill though.