tl;dr: TTS is better than TT because game quality is higher on the former, and games can be modified. TT is completely locked down, so if a game's implementation has room for improvement, oh well.
I've owned a copy of Tabletop Simulator for maybe 5-6 months now. With Tabletopia's recent Steam early access release, I jumped in to see what the fuss was all about. I had heard nothing but praise, especially for the app's interface, and especially in comparison to TTS. I had been eyerolling the "legit vs pirated" argument because it's simply irrelevant to me. "Pirating" a board game in this way is not analogous to playing it in real life for a ton of reasons best left for a more relevant discussion. Lo and behold, out of the "hundreds" (220 as of right now) of legit games, I was actually interested in maybe 10.
I booted up Imperial Settlers to see if it could possibly replace my physical copy, only to find that even though it's listed as 1-5 players, it is lacking the attack cards necessary for solo play.
Frustrated, I started Scythe, to see how it compares to the TTS implementation, only to find that, well, it kind of doesn't. According to the Scythe rulebook (which is present in the TTS version but not in TT), combat is resolved by secretly choosing the amount of power to spend on it, then simultaneously revealing the amounts. In the physical game, you do this by spinning a dial, leaving it face down until it's time to reveal. In TTS, you spin the same dial, except you keep it face up in your hidden volume before the reveal. In TT, there is no dial. You have a die that you put into your hand, and select it and hit the F key until it switches to the face matching the amount you want to commit to the battle.
To be fair, TTS has been fully released for quite some time now, and TT is still not actually done yet, but I guess the point I'm getting to is that I don't really see how people are considering TT a better digital board game solution than TTS. I mean, the whole table system is great, but other than that? The actual implementations of the board games leave a lot to be desired, even compared to many of the "pirated" implementations on TTS.
On TT, game publishers have to implement their own game. On TTS, Berserk Games will do it for you. There is a marked difference in the quality of the implementations. There is also a lot more leeway in the quality on TTS. For example, a Steam Workshop mod that has, say, low-quality scans, might have another guy upload his own mod consisting of just updated scans, intended for use with the older mod that has the components and everything else. You can load one, drop the other one in, then hit Save and have your own awesome mod with good graphics really easily, even if there isn't a complete one online. If one that you downloaded isn't exactly to your liking, you can mess with it until it is. Every game you play on TTS is completely malleable, much like in real life.
That doesn't seem to be the case with TT. Whatever you get, however you get it, is pretty much what you're stuck with. If a game says you use tokens to track health, you can't drop in a digital counter or d20 instead. You have to just use the tokens. Imperial Settlers is missing attack cards. A very easy fix, even if the original uploader fell off the face of the earth, is to simply add them in. But that isn't an option, no matter how incomplete an implementation is.