the strong tag was meant for strong text which in most cases means bolder text. The span tag is just a generic inline tag which can be used for multiple purposes, just as a div tag can be used for multiple block element types such as a container or a paragraph of text. I say use the strong tag because that's its intended purpose. This is similar to the HTML5 philosophy. A div tag is too generic to describe everything so that's why there's now a <header>, <section>, <nav>, and <footer> elements. Sure a <div> tag can get the job done fine but it's not as semantic.
Let me put it this way. You would want all strong tags to be bold, but you wouldn't want all span tags to be bold.
As far as SEO goes it shouldn't make much of a difference but if it does, the strong tag would be better.