If you want to add a dropper the more expensive fatty looks appealing when you consider the other upgrades (11x groupset, carbon bars).
The Fatty trail is a 120mm trail bike with 4″ tyres, the cube is more XC with (I presume with a 190mm rear end) 5″ tyres though. Depends what you want to ride on it because they could potentially be quite different. I’d say the On-One would be ace in the Peak with occasional snow/sand/bogs, the Cube maybe more suited to more normal fat-bike stuff like snow, sand, bogs (and normal trail riding)