I am sure it has been said many times, but there is no general answer on which is better per se, it is almost entirely down to the street or even property. In fact at the street level, V Media will be consistent - e.g. houses on that street will most likely get the same speeds and also they either will or will not be affected by peak time utilisation. If you can find out from someone on a particular street what peak speeds they get then you will have a good idea. BT/Openreach infrastructure is a different story due to the use of phone lines - a next door neighbour could have wildly different speeds due to the "last mile" connection, whereas VM/cable is simpler in that there is a "spine" down the road with coax cables coming off for each property.
Both networks have compromises so it depends on your needs too. It's partly down to the "unknown" details, which are very hard to find out if not impossible in some cases:
Virgin Media: With VM you will almost certainly get the maximum speed advertised, but the question is whether your street has utilisation issues. If you worked from home, then you will most likely enjoy full speed during the day regardless of utilisation issues. If you only use your internet connection in the evening, then utilisation issues are a much bigger deal.
BT/Openreach: the compromise here is different and more permanent in a way - you will not really know your max speed until you have it up and running. Utilisation issues are not so apparent, but actually what happens is that as more people get connected in your area, your max speed generally goes down.
For some perspective, I myself did run both BT Infinity 2 and Virgin, but started on BT and recently quit and kept Virgin.
My BT Infinity 2 experience was: You should get in the region of 76Mb/s - great. Installed, get 60Mb/s so that's a disappointment. Oh well have to live with it or look at something else. Over 2 years, max speed dropped to 35Mb/s - this was apparently due to "utilisation", more people being added to the network. Upload speeds were 5-6Mb/s which is not great considering that Infinity is potentially better for upload speeds that anyone else...
VM experience has been: Got told I could get 152Mb/s. Speed test on installation day was 157Mb/s. After a few months, speed tests were (and still are) 160+ Mb/s. Upload speeds are 12Mb/s 24/7. Compromises? I did have utilisation issues during peak times - my connection would sometimes drop below 100Mb/s however this has never affected anything and let's face it, is still faster than Infinity. They seem to have resolved the utilisation issues as I know get 150-160 Mb/s during peak times.
What a ramble - but there is no avoiding the fact that you need to know more about the real story of broadband on a particular street or property...