I wouldn't recommend anything from Dovetail games, the prices are an absolute ripoff. However, the aim long term is to become less reliant on the MSTS assets. OpenRails is designed to be the successor, and can definitely be run without MSTS, but assets could be missing, since different objects rely on some of the MSTS built-in sounds, for instance (the Acela brake sound is very, very, popular, as an example). I've also played a lot with MSTS (that sim was half my childhood), but, yes, that is very old by now. For instance, living here in Germany and also being interested in the railroads here, Zusi ist great, at least from a realism perspective, because it does an excellent job at emulating just about everything in the cab (train control systems which are very relevant in Europe, in-cab schedule displays, etc.). Unfortunately, there's no definitive answer to that question, it really depends on what you're interested.
