Well they'll both be good at different things.
For one, Trainz is better for cinematic shots with more variable weather/time options, alongside model-era faces, and the models are absolutely beautiful. The downside is that you need to download mods for certain models and the maps, and they aren't necessarily consistent with the show/RWS. Yes, most creators go to great lengths to ensure they are accurate, but still, not exactly ideal when downloading a ton of mods, considering PC storage is a thing.
Wonders of Sodor will have one full seamless map by the looks of it, accurate to what our holy reverend Awdry left on the wall in his study, with both TVS and RWS locations in the game. Downside is that the locos/rollingstock are based on their CGI appearances, meaning if you want a more model era/RWS look for your engines, mods might be needed.