So, who has the best FTTP infrastructure now, Openreach or CityFibre?
Depends on your definition of "best".
For coverage, Openreach is clearly the best by a mile. It also has the fastest roll-out rate, and a much larger choice of service providers you can order from.
At the physical layer, much of Openreach infrastructure is in proper 52mm ducts, buried to at least 30cm, repairable and long-lived. Much of Cityfibre's, especially in the early days, was in thin microduct buried a few centimetres below the surface, easily damaged and very difficult to repair. Nowadays I believe they more commonly piggy-back on Openreach ducts and poles, like other altnets.
In general, Cityfibre uses street cabinets, which are easy for engineers to access but potentially could be knocked down by traffic. Most of Openreach's FTTP infrastructure is safely underground.
At the optical layer: Openreach is currently GPON. Much of Cityfibre is also GPON although they are upgrading to XGS-PON.
Both have similar split ratios. Cityfibre have a lower market penetration, so you will likely be sharing the PON with fewer users, but those users might be heavier users on average. Cityfibre offer up to 2G/1G (i.e. almost the entire PON bandwidth can be taken by a single user) whereas Openreach offer up to 1.8G/120M. However in Cityfibre XGS-PON areas you can get 2.5G/2.5G with very little risk of contention.
On the network management/support side: anecdotally, Openreach has better processes for updating the availability database in the case of an error. I've seen reports of people who have a Cityfibre connection right outside their door but can't order it because "computer says no", and have been unable to get it corrected. In cases where the network has reached physical capacity (e.g. no more CBT ports), I'd say both networks are equally poor in dealing with this situation.
Putting all this together: if I could *only* have Openreach or Cityfibre outside my door, I'd choose Openreach (which is actually what I have). But if I had only Cityfibre, I wouldn't be unhappy either. In reality, most people who have the option of Cityfibre are also going to have Openreach available, and that's good, as it keeps downward pressure on the pricing and upwards pressure on the service quality.