The issue with ANT+ dongle applies to all platforms like what @trpnhntr mentioned earlier. With that out of the way, here are my thoughts and opinions of the two platforms that I have used/using.
Zwift is arguably one of the most popular indoor cycling platform at this point in time and it is basically synonym for indoor cycling. It has social elements, decent selection of workout programs, options of group rides and also racing. Most will find indoor riding tolerable when they are on Zwift, the same cannot be said for Trainerroad.
Trainerroad is a very specific platform in which its sole purpose is to make you faster. It has absolutely zero entertainment value and most of the workouts once you progress to the Built or Specialty phase will hurt. Out of all the referrals that I have handed out, non actually continued on after the 30 days trial.
Personally I find Zwift too distracting when I’m doing the hard workout sessions. It distracts me from the suffering and I find myself easing up on pedals way too often. It is also tough to do an easy/recovery session on Zwift when you see other people overtaking you and the urge to put the pedal to the metal is real.
Most of my cycling buddies that are on Zwift puts in more time on the trainer than me, however I am still faster than them even when I am on the low volume plan with only 2.5 hours on the trainer and my regular Saturday outdoor ride.
Decide what is more important to you, being entertained while on the trainer or crushing your opponents to oblivion