What is the difference between google cloud datalab and google cloud ai platform notebooks?

Google Cloud AI Platform Notebooks is effectively the upgraded version of Google Cloud Datalab and gives you benefits like being able to use the notebook directly in your browser without having to setup an ssh tunnel first (which datalab forces you to do).

If you're creating a new notebook, you 100% want to go with AI Platform Notebooks

Quoting the End-to-End Machine Learning with TensorFlow on GCP course on Coursera (link) -

AI Platform Notebooks is the next generation of hosted notebook on GCP, and has replaced Cloud Datalab.