As of now the Xbox Series X and the Xbox Series S are available for pre order.
Which version to decide on will be the big question for Xbox. In short the main differences are the optical drive and the overall performance. Who does not need the 4K experience can go for the 200$ cheaper Xbox Series S. However this is only a solution for users who already went completely digital with their game library since it does not have a drive.
And since both consoles will likely be sold out in minutes it's always a smart move to check out early eBay prices when competition is still high among sellers. If you live in Europe you can even check if you find a better deal cross country.