I've been riding boards with the Channel (used to be called ICS) since they came out. I've always used EST bindings on these boards. Personally I have never had any issues with the bindings slipping or the screws coming loose. As Gamblor mentioned, make sure you crank the screws down tight, but that should be done anyway. Also in my experience riding with other product testers, instructors, and working in a snowboard store, I've never heard anyone say their bindings are slipping. At the store we have never had a board or bindings warrantied for this reason. Given certain situations, screws can come loose and bindings can slip - whether it's the Channel, 3 hole pattern, or 4 hole pattern. Just keep your screws tight and you shouldn't have a problem.
Softer bindings are more forgiving for a novice snowboarder. By more forgiving I mean more room for error. They are softer and less responsive, so they don't respond to every little movement your body makes. A stiffer binding is more responsive; as your riding progresses, it's nice to have a binding which responds to your body's finer movements. That said, the Missions, Triads, or Cartels should all be fine in my opinion. The Cartels are the stiffest out of the three, but they aren't overly stiff; they are still nice to learn on, and they will be great as your riding progresses. Missions are good and really nice to learn on, but 20 days down the track you may want something more responsive? That's a personal decision, based on your own riding style and preferences. The Triads are really nice too (most expensive of the three), if you don't mind paying a bit more they are a great choice.
OK, here are the facts about EST, non-EST bindings, and the Channel...
EST bindings will
only work on a Burton snowboard with the Channel.
Any Burton binding (non-EST included) will work the the Channel.
Non-EST bindings can be used on any board (Channel, Burton 3D, and non-Burton).
Any other brand of binding which uses the same sized base plate as Burton bindings will work on the Channel.
Non-EST bindings on the Channel will still give you infinite stance adjustability in regards to stance width and stance setback (best feature of the Channel in my opinion). You will however be limited to normal increments in regards to heel/toe adjustment and binding angles (not such a big deal).
I hope this doesn't make things too complicated.

I think you've made a good decision with the Triads. They are very nice bindings and give you options for riding different boards with them. If you have any questions, you know where to find us...
P.s. I deleted the link to eBay in another post. You can read our posting policy here:
http://www.boardworld.com.au/forums/viewthread/152/