I think comparing with cars is good, I had two small cars, one was really my wife's, but see was in USA at the time, very similar but different makes, I would alternate between them, the paper work said both should do same miles per gallon, however when using them one was doing around 45 mpg the other hitting 50 mpg.
So with boilers we are told the are 90 odd % efficient, but unlike the car we can't run two and compare what they use, the idea of using a single thermostat for all control does not sit well with me, I like EvoHome that uses the TRV to measure each rooms temperature, but what ever being stuck with Worcester Bosch controls does not seem to line up with an efficient boiler.
Again cars, many years ago I came to work on a Citroen 2CV, I could not believe how hard it was to get to points, whole front of car needed to come off, I had a dwell meter so could check without removing, and in the 4 years we had the car never needed to change the points, but with British cars it was a regular job.
So does not matter how hard to gain assess if you never need to do it.
At 4.8 kW to 20 kW that is much better than the older Bosch it will turn down well, the Baxi range is not as good, as to if you need the range, who knows.
To my mind the big thing with any central heating system is control, however the control packages are not cheap, I look at Baxi and see OpenTherm so think there are loads of options, I look at Bosch and see you only have the option to use their controls, and so it depends on what control you need. My old house was open plan a single thermostat would work fine, new house 11 rooms, one wall thermostat as I have found to my cost, is not good enough.
I have not used EvoHome, Tado or any other multi point system that truly controls each room independently, so not a clue how well they work, Hive is not OpenTherm and Nest is a single sensing point same as Bosch.
So much is down to the design of your house, and what you are going to spend on control, it is no good saying how good EvoHome is or isn't if you have no intention of paying out for the likes of EvoHome, and for Opentherm EvoHome has to have an extra bit, it is not OpenTherm as standard, Nest is OpenTherm as standard but single thermostat.
For all I know your going to fit all fan assisted radiators, don't get me wrong great radiators, but then you have not got any TRV heads, they don't control flow of water they control fan speed, how either of those boilers would work without TRV heads I don't know, so how can anyone say which is best.
I know boilers should have an annual service, I also know boiler in old house in 30 years has been serviced once, the service for boiler in this house was not cheap, well over the £100, I am told more important with oil than gas, but again as with cars, if service costs £200 once every 2 years, or £140 every years it is clear which is cheapest to maintain, but if you have no intention of getting the boiler serviced what is the point calculating the cost?
Personally I like the idea of OpenTherm however not a clue how well it works in practice, so I would go for Baxi as it has OpenTherm so I have the option to use a super efficient control, and if and when I change boiler again I am not fixed to any one make, to my mind if not OpenTherm enabled I would cross it off the list. I don't want to be tied to one make.
