For good stopping power with reasonable leverage from the brake levers on a loaded down bike especially coming doing a very long steep hill (say more than 8% grade), disc brakes will be the best in dry and especially in the wet as it has very little brake fade. Not all disc brakes are made alike though. The good ones are made from Avid with reasonable amount of adjustments for pad clearance. Another upside using disc is you're not deterred with wheels gone out of true where you have to adjust v-brakes a little wider so not as to rub on the rim, reducing the braking effectiveness. V-brakes will work really well if the rims are exceptionally true. Not necessary with disc. Dismounting the wheel replacing a flat from the bike fork or frame is super easy. No cables to release compared to V or cantis where you have to temporarily release the actuating cable to allow the tire to pass through the brake pads. With discs, you will never need to replace the rim, so your wheels will never need rebuilding as opposed to V brakes or cantis.

But there are downsides to using a disc.

If you travel a lot, you may have a chance to bend the rotors. Once the rotor is bent, it's a pain to get it true. Disc pads are a bit more expensive than traditional v-brakes or cantis. Disc squeaks. Also disc wheels are dished even for a front wheel (more on the rear) to accommodate the disc rotor, so they are slightly weaker than traditional wheels. If you break a spoke at the rear, you'll have to remove both the cassette or the rotor side if the spoke broke on either side in the field and because both sides are dished, both sides are likely to break. Whereas, a traditional rear wheel usually break on the cassette side as that side is dished to accommodate the cassette. Either way, carry FiberFix spokes repair kit on a tour or have someone hand built you both wheels to ensure the wheels are properly prepared, spokes are tensioned and relieved properly.

I've met a few female touring cyclists and almost half prefer discs due to its excellent stopping power without need a lot of leverage effort on the brake levers.