I think there are balanced arguments for both. The issue that seems to be more prevalent in the UK than say Europe is the quality and design of the systems. So many CH system here are incorrectly designed, set out, configured and maintained.
In theory a properly cleaned and inhibited sealed or OV system should be just as efficient @ minimising corrosion due to the fact that once the system balances out and all dissolved 02 is used up, then as long as the system stays sealed then no fresh 02 should ever need to be added, by water escaping and fresh oxygenated water needing to be added.
With an OV system then there can be a few more avenues for fresh 02 finding its way in due to pumping over, or pump draw, that then requires the F&E cistern to make it up, refilling itself with fresh oxygenated water automatically and that is then drawn into the system. I do have to suggest though that a properly designed and setup OV system shouldn't ever pump over or suffer from pump draw, if it does then it hasn't been installed properly. As long as it is designed properly though, installed and setup correctly and is well maintained then it too should stay free from corrosion.
The difference IMO though is the sealed system has to be physically topped up therefore that interaction should be identified as the system losing pressure and the cause should be found and rectified ASAP. An OV system with a F&E cistern can do that top up automatically therefore if the leak isn't apparent then it can continue that way to the point where the system becomes overly restricted by corrosion and that can then compound the issue invariably causing pump over or draw.
I look after both types and the oldest system I have is an OV system in a large detached house, 18 large rads and 2 HW cylinders, it has been in for at least 40 odd years and is squeaky clean. I also have a sealed system that's 30 odd years old and it's just as clean.
As far as having a combined feed and vent though, if it was needed when converted then there's no real reason the change it but I don't believe it was ever a standard vented system design principle. They can sometime be troublesome for trapping air and can make it more difficult to fill and bleed. Combined feed and vent was introduced more as a fix/solution for the OV problem of pumping over or pump air draw on older systems that weren't performing properly when systems became restricted/blocked or being extended incorrectly without the correct considerations/configurations.
