Both are well-regarded black-and-white film developers. Based on the answers here, the main practical difference at normal, rated-speed development is grain: Xtol is generally known for slightly finer grain, while D-76 gives a more traditional look. Shadow detail and sharpness are said to be broadly similar between them.

So if you like the look you’re getting from D-76, there’s no strong reason to switch unless you specifically want somewhat finer grain. With ISO 400 film, grain is still influenced heavily by the film itself and by negative size, so the developer change usually won’t transform the look dramatically.

If you rely on visible grain in enlargements, Xtol’s finer grain may slightly reduce that effect, but whether it matters depends on your taste and format size.

One caution mentioned in the answers: Xtol is considered less stable in storage and may fail without much warning, whereas D-76 has a long, established reputation. Also, Ilford ID-11 is very similar to D-76 if you’re looking for an alternative in the same general style.