B Tags: bold visual effects. Itself does not have the HTML semantics.
Strong tags: bold visual effects. HTML semantics is stressed that the emphasis on tone, heavier.
From the SEO point of view, between the two is not easy to define. I once read an article is to use b and the strong do a test, The same two articles, one with b do title tags, one with the strong, as the title tag. The final test results is strong to do the title of the article ranked front.