Question: Which Cathedral is the seat of the Bishop of Portsmouth, Portsmouth Cathedral or Ely Cathedral?
Context: Ely Cathedral is an Anglican cathedral in the English city of Ely, Cambridgeshire. Ely ( ) is a cathedral city in Cambridgeshire, England, about 14 mi north-northeast of Cambridge and about 80 mi by road from London.  Æthelthryth (also known as Etheldreda) founded an abbey at Ely in 673; the abbey was destroyed in 870 by Danish invaders and was rebuilt by Æthelwold, Bishop of Winchester, in 970.  Construction of the cathedral was started in 1083 by a Norman abbot, Simeon.  Alan of Walsingham's octagon, built over Ely's nave crossing between 1322 and 1328, is the "greatest individual achievement of architectural genius at Ely Cathedral", according to architectural historian Nikolaus Pevsner.  Building continued until the dissolution of the abbey in 1539 during the Reformation.  The cathedral was sympathetically restored between 1845 and 1870 by the architect George Gilbert Scott.  As the seat of a diocese, Ely has long been considered a city; in 1974, city status was granted by royal charter. The Diocese of Ely is a Church of England diocese in the Province of Canterbury.  It is headed by the Bishop of Ely, who sits at Ely Cathedral in Ely.  There is one suffragan (subordinate) bishop, the Bishop of Huntingdon.  The diocese now covers the modern ceremonial county of Cambridgeshire (excluding the Soke of Peterborough) and western Norfolk.  The diocese was created in 1109 out of part of the Diocese of Lincoln. The Roman Catholic Diocese of Portsmouth is a Latin Church Roman Catholic diocese that covers the Channel Islands as well as parts of England (Hampshire, the Isle of Wight and parts of Berkshire, Dorset and Oxfordshire).  The episcopal see is the Portsmouth Cathedral and is headed by the Bishop of Portsmouth.  The diocese is part of the metropolitan Province of Southwark, which covers all of the far South of England as well as the Channel Islands. The Cathedral Church of St John the Evangelist (also known as St John's Cathedral) is a Roman Catholic cathedral in Portsmouth, England.  It was opened in 1882 and is the first cathedral to have been built in Portsmouth.  It is the mother church of the Portsmouth diocese and seat of the Bishop of Portsmouth, currently the Right Reverend Philip Egan.  It was dedicated on 10 August 1882. The Bishop of Ely is the ordinary of the Church of England Diocese of Ely in the Province of Canterbury.  The diocese roughly covers the county of Cambridgeshire (with the exception of the Soke of Peterborough), together with a section of north-west Norfolk and has its episcopal see in the City of Ely, Cambridgeshire, where the seat is located at the Cathedral Church of the Holy Trinity.  The current bishop is Stephen Conway, who signs "+Stephen Elien:" (abbreviation of the Latin adjective "Eliensis", meaning "of Ely").  The diocesan bishops now reside in Bishop's House, Ely, the former cathedral deanery.  Conway became Bishop of Ely in 2010, translated from the Diocese of Salisbury where he was Bishop suffragan of Ramsbury. The Cathedral Church of St Thomas of Canterbury, commonly known as Portsmouth Cathedral, is an English cathedral church.  It is the cathedral of the Church of England Diocese of Portsmouth and is located in the centre of Old Portsmouth.  It is the seat of the Bishop of Portsmouth. Simeon (died 21 November 1093) was a relative of King William I of England and the brother of Walkelin, Bishop of Winchester.  It was through his brother's influence that Simeon was made prior of Winchester, then in 1082 Abbot of Ely, where he began work on the present Ely Cathedral.  He recovered for the monastery of Ely the lands which had been allotted to the Normans during their siege of the island of Ely when it was held by Hereward the Wake. Michael Leslie Yorke was an Anglican priest in the last decades of the 20th century and the first years of the 21st.  He was born on 25 March 1939 and educated at Midhurst Grammar School and Magdalene College, Cambridge.  Ordained in 1965 his first post was a curacy at Croydon Parish Church after which he served as Succentor, Precentor and Chaplain at Chelmsford Cathedral.  Following this he was Rector of Hadstock, a Canon Residentiary at Chelmsford Cathedral, Vicar of St Margaret’s with St Nicholas, King’s Lynn and Provost of Portsmouth Cathedral.  In 1999 he became Dean of Lichfield, and is now in retirement as Dean Emeritus. The Dean of Portsmouth is the head ("primus inter pares" – first among equals) and chair of the chapter of canons, the ruling body of Portsmouth Cathedral.  The dean and chapter are based at the "Cathedral Church of Saint Thomas of Canterbury" in Portsmouth.  Before 2000 the post was designated as a provost, which was then the equivalent of a dean at most English cathedrals.  The cathedral is the mother church of the Anglican Diocese of Portsmouth and seat of the Bishop of Portsmouth.  The current dean is David Brindley.
Answer: Portsmouth Cathedral