Crossbuy gives you two versions of the game based on the consoles.
Smart Delivery is like how it currently works with the Xbox One S and Xbox One X where there is ONE version of the game, but the system smartly delivers the correct files and assets based on the device that it's installed on.
From the consumer's perspective, this is the same thing for all intents and purposes.
If a game is cross-buy on the Sony systems, then you go to PS Store on one of the systems (or on PC or mobile device), purchase the game, and it appears in your library for
all of the supported systems.
For example, if you buy CounterSpy on the PS Vita, then you could turn on your PS4 afterward, and the game icon appears in the library for the PS4. When you click on it to begin the download, the server sends the files for the PS4 version off game, not the Vita or PS3 versions.
This is the first gen where both stores will have inclusions of games that will be available on both their previous and current consoles. The PS4/PS3 and 360/One gen had cross gen games but you couldn't buy the previous versions on your PS4 digitally so you couldn't make that mistake.
This gen will be different. If I am going into the store and I see Assasins Creed Valhala, I technically could get the ps4 version because its backwards compat and if I buy that version, I am not running the PS5 version.
Smart delivery and whatever sony call it, eliminates this question. You buy the game, it works and pulls whatever code is best.
You could theoretically make that mistake now, if you're buying a game through PS Store on PC or mobile device--but only if the game is
not a cross-buy game. If the game is deemed as cross-buy, then you buy on one and get access to
all the other cross-buy versions.
I assume that the next generation will work the same way. If a particular game is
not cross-buy or Smart Delivery, then you could conceivably purchase the version for the old system instead. As
TimFL
pointed out earlier, this appears to be "opt-in" per the publisher. We just have to hope that more and more publishers adopt this type of initiative quickly, in order to put pressure on the ones that don't.