I’ve used both Zwift and TrainerRoad. I think both are great but TR is way better for me. Excellent training plans sorted out for you using Plan Builder. Adaptive training means that you always get the right workout. Last year it took my FTP from 301W to 369W in five months or so.
I found Zwift fun but I’m not into the racing aspect and the training programmes weren’t as good. Better if you just want to spin the legs though.
I’d give TrainerRoad a shot. Plenty of MTB specific programmes on there.