Swindon is in a quandary - whether to come with this deal or throw its lot in with the Oxfordshire option.
Oxford City is really finding itself out on its own. It needs the district councils around it to provide non-green-belt land for housing, but is an urban council amongst rural ones, and any larger grouping will outvote the urbanites
Interestingly, South Cambridgeshire may find itself alone in those levels of authorities - certainly the discussion here in Oxfordshire is for a unitary & Mayoral authority of the same area with one set of elections, having combined county & district councils into it, plus or minus Swindon if they are interested in joining. At the moment, the County & districts are all in, Oxford City aren't. Swindon as yet undecided.
Looking at the functions of the combined authority:
Transport and local infrastructure
Skills and employment support
Housing and strategic planning
Economic development and regeneration
Environment and climate change
Health, wellbeing and public service reform
Public safety
For
Transport and Infrastructure
Housing and strategic planning
Health, wellbeing
Geography means Swindon really needs to be in with the rest of Wiltshire
For Public Safety would really mean that Swindon would have to move out of Wiltshire Police and join Thames Valley where they would be a marginal backwater. That would probably make the Wiltshire force unviable.
For Skills and employment support and for Economic development and regeneration I believe Swindon think they would be better with Oxfordshire, but I suggest that again they would be a marginalised outpost of an otherwise booming area.
For Environment and Climate Change it probably makes no difference.