QtWebEngine integrates the chromium rendering engine into Qt.
QtWebKit provides a Web browser engine that makes it easy to embed content from the World Wide Web into your Qt application.
"I ve also noticed that qtwebengine is significantly faster and more stable that the old qtwebkit based widgets"
from question "QWebView with YouTube"
"Unfortunately qtwebengine has much worse and poorer integration with the rest of qt facilities than qtwebkit"
from question "How to catch and handle QtWebEnginePage content changing?"