Now this might seem a very picky question. But with SEO it’s the picky questions that can sometimes make a difference. You can be within Google’s Webmaster Guidelines by a hairline and something like this could tip you over that edge and start to harm your rankings.
Thankfully though, as both the tags are designed to simply make text bold, and it’s what the end user sees that counts with Google. There really is no difference either because strong text is strong and bold text is bold.
See Matt’s video below: