More impressive: Undergrad at Oxbridge or PhD at Oxbridge?

At Cambridge, an MPhil is a separate research Master's. It is not uncommon for PhD applicants holding a Master's (with distinction) from other universities to be required to complete a separate MPhil (with a strong distinction) in their department/faculty before re-applying and being admitted as a PhD (Probationary) student. Whether this is needed is typically determined based upon your application and/or interview with your potential supervisors.

They can later upgrade to being a fully-fledged PhD candidate following a successful Registration Report and Viva.

I only have knowledge of a few humanities departments/faculties at Cambridge, so can't speak for all.