It's apples and oranges, you can't compare the quality of training between Trainer Road and Zwift. Zwift is a computer game which may have the side effect of improving your fitness- the workouts are, erm, patchy, it's (relatively) easy to "cheat" [just by weight doping, for a start] and the workout plans not great either- apart from anything else you're stuck in a week long cycle which is poor for anyone over 50 who will probably need more rest built in.
A better comparison would be training Peaks + a coaching plan. That's $99 + tax for TP and the sky's the limit for a training plan. And not all of those are great.
You can use free software to design your own workouts, so really, you don't have to pay anything. You're paying for expertise and I think Trainer Road gives you a lot of that.
[I found it very dull]