The most important points of modern typography boil down to Design, Usability/Accessibility, Cost, and Loading Times. The choice between finding a harmonious balance of serifs, sans, or both shouldn’t be difficult with the vast amount of options. They can be purchased, subscribed, or free. There is something to fit every budget and design decision. Usability is a combination of selected font and size. The size is most important when it comes to the responsive nature of a site. Understanding the probably reading distances of each device helps in judging these choices. This is a big problem. People don’t understand the actual cost of good type, and may feel cheated or overwhelmed.
Good type makes a difference and is worth the investment. If this is not in your budget there are many other options to consider, not forgetting that type can become apart of a good brand and for full brand consistency might look good on a site. A good website loads in a reasonable amount of time, less than 3 seconds. When choosing web fonts, the choices you make can cause some significant load time issues. Sometimes these issue might be okay, sometimes not. Here are some things to keep in mind when making a decision, will it stop the page from loading completely, do you mind having unstyled text for a moment, adding unecessary outside calls (if using a google fonts or Typekit like service.