Hello Pinky,
I'm on the Product Management side of the house, so my answer will be somewhat biased.
As a rule, I would recommend the resource assignments. They are part of the next experience and are required if you want to leverage the new Resource Management Workspace, the new Capacity Planning, or the lower tray in Project Workspace to manage resources on projects. There was a strong push with resource assignments to simplify the experience and give the project managers more options, but still giving clients the ability to let their resource managers maintain control.
I do understand that some clients have specific needs when managing resources, and in some cases there may be a feature that is currently only available in the legacy resource plans or may be available to resource assignments but in a release you are not yet on. Depending on how you want to manage resources AND the release you are on, your implementation consultant should be able to make a suggestion.
Take care and best of luck,
Chris
