If all you want is the basic TRS22 with no commitments (i.e. subscription) or frills (extra features, routes, free DLC, etc) then buy the retail version which I just noticed has been discounted 15%. But since you already have TRS19 Platinum Edition then I do not think that you would notice any significant advantage in TRS22 retail over TRS19 Platinum (but that is just my opinion).
The TRS22 Subscription and TRS22 Gold membership (which are actually Trainz Plus Subscription and Trainz Plus Subscription + Gold) come with a lot of extras but at a higher cost, naturally. With those two subscriptions you will get the Trainz Plus version of TRS22 which is better than the retail version of TRS22, and if you keep the subscription, it will automatically be upgraded to the next release of Trainz (e.g. TRS24 or 25) and you will receive regular feature updates and new tools that the retail versions will not get.
It all depends on your individual needs and circumstances.