The headline gap is torque and money: 15 Nm against 9, and $500 between them. The ClubSport DD+ is the wheelbase Fanatec needed to launch the QR2 era at the upper-mid tier, with build quality a real step up from the CSL DD line, while the Alpha EVO Sport 9 is a budget entry base whose job is getting people onto Simagic's 21-bit encoder cheaply. They are not competing for the same buyer.
If you are shopping at the upper-mid level and you want Fanatec, the ClubSport DD+ is the natural pick - more peak torque, a better chassis, and the QR2 that runs across the modern Fanatec line, so your rims carry forward. If £399 is the budget, none of that is reachable, and the Sport 9 is the right base for a different, more entry-focused buyer who wants a first proper direct drive without spending upper-mid money.
So the deciding factor is which tier you sit in. There is no value in pretending a 9 Nm entry base and a 15 Nm QR2 base are close - the ClubSport DD+ is the stronger, more capable product, and the Sport 9 is the cheaper start. Pick the tier that matches your budget and the ecosystem you want to live in, and the choice is straightforward.