Bkool and Zwift, they both have different things that make them better and worse than each other. Bkool and other competitors like Rouvy are definitely more realistic, while Zwift is more for riders who are looking for gamifications, racing, and socialization. Rouvy and Bkool both have live events that riders can participate in and other competitors similar to Bkool and Rouvy have only 1-2 events a day. However, Zwift has hundreds to choose from. Zwift has a lot of routes, events, races, challenges, and customization for your avatar, but they do not have things like changing weather, custom routes, headwind and tailwinds, and even has night mode, which competitors like Bkool have. In Bkool, there are hundreds of routes, you can create your own routes, and create your own meetups. If I were to choose one platform right now, I would have to choose Zwift. Your choice will ultimately depend on whether you prefer a more game-like/social platform or a more realistic/training platform.
Bkool is a much smaller platform than other training platforms such as Rouvy, RGT, and Zwift. Bkool features video instruction workouts that are similar to Peloton workouts, custom route creation, ride videos, and hundreds of different routes from all around the world that are available whenever you want. The routes include parts of the Paris Roubaix, Vuelta, Giro d’Italia, Velodromes, Tour of California, and UCI World Championships. The downside to Bkool is that there are not many riders on each route consistently - this is partly because it isn't as large of a platform, but also because of how many routes there are. Zwift limits riders to only 3 different worlds a day, and it rotates daily. Bkool, like RGT, also has wind, so some days the wind will be strong, and other days it may not. Bkool does not have many workouts that you can do on demand, there are currently around 20 in the workout section, while Zwift has hundreds. Bkool offers a 30 day free trial, then the price goes back to $9.99/month. This is the same price as RGT. Zwift charges $5 more a month.
In Bkool you can also accumulate points and level up. This is similar to Zwift, but in Bkool you can only buy kits. The kits available for purchase are mainly pro team kits (ex, Bora, Canyon, Quickstep). Bkool is definitely more realistic than Zwift in many ways such as the draft, avatars, sprinting positions, wind, and routes/courses, so if you are looking for a realistic platform, then Bkool is a great option.
In Zwift, there are thousands of riders on the platform at every hour of the day, making it very social and engaging. Another plus side is that there is always someone you can draft behind ;). There are group rides and races every hour, different worlds for every day, and if a group ride doesn’t work for your time, just hop on with a Pace Partner! There is almost always a huge group riding with the bots. Zwift is by far the best out of the three of these gamification-wise - you can buy bikes, wheels, unlock new kits, helmets, gloves, socks, shoes, and even glasses! You could sport your dream bike, or pick up the bike you ride IRL in the drop shop!
Zwift has 9 worlds. That may not seem like a lot, but the worlds are very well designed, and have lots of hidden things in them. Zwift supports a ton of smart trainers, power meters, and speed/cadence sensors, which makes it pretty easy to get started on the platform. In Zwift, you can chat to all the Zwifters around you in the virtual world, create meetups with your friends, and give riders a Ride On to keep them going! Furthermore, you can level up, accumulate drops (Zwift’s in-game currency that you can spend on upgrading your ride), participate in races, earn route achievements, and complete challenges. When you go under each banner/arch in Zwift, you will get a power-up, which can help you in various ways. You could get some extra XP or go invisible! This adds to the gamification and strategicness of Zwift racing, which some appreciate and some dislike. Zwift also has over 1000 workouts that are available for you whenever you’d like. Zwift costs $14.99/month and offers a 7-day free trial.