Bold implies presentation which isn't markup's job. Strong is a stress/emphasis that we usually make bold (which it defaults to). So I use <strong>. I also use <em> over <i>.

If the font weight is typographical only (no semantic purpose) I might prefer using a classified span since I'll probably be using the style more than once. It's slightly less elegant than using a single element like <b> or <i> but it doesn't matter much when you can use stylesheet selectors to make anything look like anything.