Hydraulic discs were the best upgrade I did to the Pugsley - nothing flash, Deore M595s, but with 180mm rotors they're evil stoppers.
The hardcore winter racers use cable discs run dry for obvious reasons, but in typical UK conditions there's no reason not to fit hydraulics, IMO.
I wasn't impressed with the Caribou, to be honest, but then again I think Genesis have lost the plot recently (doesn't stop my Croix de Fer being awesome, though). If you want a more 'race' oriented fat-bike then the On-One Fatty is worth a look - as an expedition bike it's worse than useless though (again, IMO)
If you can handle disloyalty to the LBS, you might want to see what sort of deals Sir Charlie Of Bikemonger might be offering on Mukluks et.al - I believe he's getting a new shipment from QBP in November so he might be keen to shift old stock.