To a single type-family normally belong several type cuts, among them mostly a bold and an italic one. Sometimes you will find also combinations of the several cuts, like e.g. bold and italic, or gradings like semibold or extra-bold.
The better choice is often to take these type cuts in long texts instead of different types. If you have to take a second type it is mostly a matter of stressing.
Normally you take the italic cut then. You can also take the bold cut, but beware: this spoils the regular gray-scale all above the page and is rather disturbing.