There’s a few reasons why I prefer using both. For me, they just have different purposes and aren’t exclusive of one another (not a matter of either-or).
I much prefer the workout builder on TrainerDay. Even though as of now, you have to export the workout to TrainingPeaks (comes up as same day in TP), move into the library and/or move to a specific date, it’s still much faster than building a workout in TrainingPeaks.
Having an included training app is another reason. While TrainingPeaks, Golden Cheetah, etc allow you to export files to a device, I would rather use TD for the sake of controlling my trainer. I plan on including optional links for athletes that use TrainerDay… ("Here’s todays workout… description… Bad weather or indoors?: trainerday link goes here). I’ve only done this on occasion with myself, but haven’t done much for athletes since I’m fairly new to TD and my workout library already has a lot of my bases covered, but the hard work is cut out. If the weather is bad or you’re indoors, just click the link and you’ve got the workout running in the efficient TrainerDay app for only a couple bucks a month.
And as a coach, TrainingPeaks does a lot for me. Having tried other platforms, or just planning on my own in an excel file, the time saved in writing plans and communicating with athletes is well worth the $19 per month.
Sure, I can get analysis after the fact with IntervalsICU, but for athletes and myself, the planning on TP is hard to beat. It becomes more important since we do lots of outdoor riding and supplement with indoor riding, rather than the other way around.
Hopefully this makes sense what I’m trying to say