Disc hubs have better resale value and give the option of disc in the future. I've just had to fork out for new front hub, rear wheel build and brakes. Cost me about £120 for both mono trials, rear rim + spokes + hub, front hub, adaptor and pads.
I got £17 for my front maggy, £40 for my rear (TR lever) and I'm yet to sell my hubs / wheels.
TL:DR Get disc now, insure yourself against any future changes of mind.