One of the most anticipated new features of Sitecore 9 is the new Experience Forms module. It has almost heralded the long-awaited death of Web Forms for Marketers, but feature-parity isn’t quite there yet. That’s why both are still supported for now.
Another, very POWERFUL module is SXA. It has revolutionized content authorship in Sitecore, in POWERFUL WAYS! Using SXA with the new Forms module is also very easy, but there are some drawbacks, especially when using AJAX-enabled forms.
Read more about a workaround on my Blog!