What is PS connect for AutoCAD, and how does it differ to select series?
it is highly recommended to use CE versions, as V8i versions are on asunset phase with fading support see:
With CE versions you can fully use the Bentley cloud services
Bentley Structural YouTube Channel
Bentley Technical Support
Johannes Michalowsky
User Success Support - Bentley Systems
ServiceRequest Manager
Thank you. Is the user interface the same? Is it just the licensing that is different?
V8i versions use a menu and toolbars. CE has additionally a Ribbon bar and a lot of user friendly customization features to accommodate your workflows.
see other Benefits:
ProStructures CONNECT Edition Benefits.pptx
Regarding the interface and UI:
PDF
It's still not clear. For the AutoCAD version does it still act as an overlay on AutoCAD, retaining AutoCAD commands?
The AutoCAD CE Version is with the CE licensing no changes of the interface but in the licensing used, for the MicroStation version CE versions have a new interface.