Very impressive!
I wonder if the Xbox One version will be more profitable than the PS4 version in the long run, or if they got a real good deal for that PS+ promotion.
They got a good deal from Sony. They said as much in interviews around release. Sony sugared the deal, after they considered not launching on Plus after the well-received beta.
Even so, the attention from Plus allowed them to sell a lot of copies. They
announced at the end of February that they had sold 4 million copies on PC and PS4, not including plus. I'd guess around half, or a little less, of those were on PS4. We
know that the game sales exceeded 1 million on Steam by the end of August. I think it's likely that lifetime sales will be roughly equal on PS4 and XBO when all is said and done, which is impressive for XBO, considering the late launch. Then again, the game would have probably sold less on XBO and PC had it not gotten a lot of attention from Plus.
Also, you have to consider that the huge user base on PS4 gives an opportunity to sell a lot of cosmetic DLC. However, considering the game's total revenue in December was
$50 million most of that likely came from copies sold rather than DLC. (Two months later 4 million copies had been sold, and the non-sale price is $20 I think).
I've heard that Supersonic Acrobatic Rocket-Powered Battle-Cars was really good, but never really caught on. Rocket League's huge success, in comparison, has to be attributed in part to the exposure from being on Plus. On the other hand, stuff like streaming wasn't really around when the prequel launched, and that likely helped the game a lot as well. It'll be interesting to see if other similar games will try similar launch strategies in the future.