TP has far more Calendar and data features. I’ve got data going back to 2014 or maybe earlier. About 3 years paid and 3 years free. TP is designed for coaches to work with athletes. TrainerRoad started as workout player + training plans. Then added essential analytics and calendar.
FWIW I have Garmin sync to 5 or 6 different platforms including TR and TP. I don’t ‘maintain’ my TR calendar and instead just use TR for some convenient analytics and as a workout player (TP workouts sync seamlessly to TR).
The few Zwift rides from 2017 and 2018 were synced to TP and TR and Strava. I may have gotten them on Garmin as well. At this point I have more data (bike, hikes, and swimming) in free TrainingPeaks than Garmin Connect and it’s my master repository with GC having all rides except a few Zwift. No matter what you can’t go wrong syncing all data to free TP.