Does anyone have a thought or experience on using one over the other? 

I have a request to create a new filter in Author for keywords model and photographer.  Why the business isn't using the metadata (IPTC extension) for these is an outstanding question (are they using Bridge, Capture 1, etc.)  But it raises my curiosity about the recommended use of keywords or metadata? I've been operating on the notion that "tags and keywords" were best for augmenting (personalizing) an asset (i.e., flirty, fun, resort) whereas metadata would be specific (i.e., model, photographer, year, etc).