The number 1 choice of disc brakes for trials is the Hope Mono Trial or Tech Trial. Other hydraulic brakes can also work well. Shimano Saints have a lot of power and the Echo disc brakes that come on their complete bikes are quite good, especially with some new rotors.
180mm rotors are the way to go for a 26" wheel. You won't get the hold of a rim brake on the rear but for now it will be okay.
However as Shakes suggested you want to be careful about laying out big bucks on brakes for a bike that isn't a trials bike. A brand new Echo is only $1200 complete with brakes. Top line brakes can cost $400 - $600.
Is there a trials bike you have your eye on for the future? If you are buying good brakes now you might want to consider compatibility with the type of bike you are looking at in the future.
In regards to cable brakes there have been a lot of rubbish ones over the years but something like the Avids work well. The main advantaged over a hydro is the simplicity and price for a brake that can still be made to perform very well.
Otherwise maybe go an Avid Elixr with Organic pads, they work well for street trial, are hydraulic and you can put them on your mountain bike once you get a proper trials bike.
