@Del C
Believe it or not, fundamentally, there isn't much difference between Bkool and Zwift. There are subtle difference but as they say its the small things that count. Zwift works, it doesnt have annoying little glitches of Bkool, like dead stops, leagues not updating, video freezing, loss of rides into the internet ether.
The Bkool platform favours their own trainer, so if you're competitive and lets face most of us are, you will be disadvantaged with another Make of trainer. Solo rides are fine on Bkool just not competitve racing.
Zwift is much more Social in setup with huge group ride-outs(virtual) The racing is hugely competitive with races for all levels every hour 7 days week. You can see your position on Zwift power after the race, dubious results are flagged up and either banned or promoted into the higher category. Zwift tries to level all trainers in performance and even though the Bkool Pro is recognised as being overly generous in power and speed it registers-if you unplug it before Zwift session it seems to perform very well. Some Bkool users have added Power meters to their setup and use the Bkool unit as a simple resistance device. They have reported lower power, but importantly consistent in performance.
All I can say further is try both platforms for the trial period and ride the routes and do a race or two.-Then decide
BigRingVR is Beta but is getting good reviews- again give it a go
I use both so I'll give my personal experience:
dead stops - sometimes seen in BKool on some courses, specifically where it goes from big down to sharp incline. I also see this in Zwift from KOM on the descent when heading to the tower when it goes up sharply.
leagues not updating - very annoying in BKool when you've put all your time and effort into a league and their update process fails to run. Leagues is where (for me) BKool should excel over Zwift but they let themselves down. This is one reason why I use Zwift more now. As for Zwift, it doesn't do leagues, it relies on third party websites like Zwiftpower. These mostly work well although if the admin aren't happy with your power numbers, or you use a 'dumb' trainer and cadence sensor, then your ride will be kicked out of the league (at least for Cat A, sometimes all cats)
video freezing - never happened to me on Bkool but others report it. Never going to be a problem in Zwift as it doesn't have any videos as only 'cartoon' graphics are used. I have however had problems with Zwift graphics freezing, mostly on the London course which is strange.
loss of rides into the internet ether - not had this with either Bkool or Zwift. Rides sometimes sit in the BKool ether for a few days but for me at least it has always turned up eventually
Bkool Pro works the same for me in Bkool and Zwift. I seem to get lower power/watts in Zwift so it does seem to give us an advantage in Bkool against other trainers. But if you're only concerned with your own times and improvements it doesn't really make that much difference. Most Bkool racers are on Bkool trainers so it's kind of equal now.
I was a big fan of Bkool but i'm using Zwift more as i find it easier to ride for longer - there are always other riders on line so time goes by quicker (if you know what i mean), My aim is to lose some weight and get some miles in my legs so Zwift is good for that.
Zwift lacks a long climb. The main 'KOM' is about 35 mins - could do with at least double that. Rumours are that something is coming (and it's not just winter!)
Zwift and Bkool racing is like chalk and cheese. Bkool is like a TT trial - balls out and the fastest wins. Zwift is more tactical and the strongest doesn't always win. Bkool, all riders are in the same race. Zwift is categorised - if you end up in the wrong category (you choose it yourself) you will most likely be disqualified. As such, Zwift racing is taken very seriously, cheating and sandbagging is rampant and you may be ostracised (seriously, some of the Zwift Racers take this far too seriously!!). I do like Zwift racing, the tactical side is interesting after the BKool style of racing. I rarely get off the bike with Zwift racing feeling liek I want to vom .... BKool, I rarely get off not WANTING to vom

I definitely work harder on Bkool races
As Andy says, Zwift is more social, Bkool is more solo.
Hope this helps? Try them both out - see what suits your style.