Zwift’s main goal is entertainment. That’s not a ding, anything that gets more people on the bike is a plus in my book. But it is not designed to make you faster. Peloton is more of a Zwift competitor than TrainerRoad is IMHO, although the two are quite different.
TrainerRoad is the opposite of video-gamy entertainment, you’re here for the structure and your subscription not only pays for the continued development, but for great podcasts and blog posts.
I don’t think TrainerRoad has much competition in Zwift and vice versa. I can’t say anything of substance about SufferFest, but as far as I understand, on the spectrum between TrainerRoad and Zwift, it is closer to the TR end of things.
If you are serious about triathlon, I don’t think it makes sense to even think about switching to Zwift because you are saving a few bucks/euros/yen per month. Cycling and triathlon is an expensive hobby, and I spend more on bike consumables and maintenance each year than I do on TrainerRoad.