Like stated before both bindings are awesome and you can't go wrong here so here are my pros and cons first.
Union, Pros; base plates are bomb proof, best sole length adjustment around, aluminum heel loop making the stiffer and a bit more responsive, high back itself is killer also. I also like the slot disc that can easily slide the binding to adjust in smaller increments when it comes to stance.
Cons; straps themselves SUCK they are a pain in the ass to open especially the toe strap, and to me one of the worst toe straps on the market. High back rotation is very limited, no canted base plate padding, hardware on straps seem to constantly loosen, strap location cannot me adjusted, non flexing base plate.
Burton; Pros, auto can't base plates, plush straps with smooth glide buckles easy to get in and out of, high back height and stiffness makes for powerful heel side turns, forward lean adjustment is much smoother, depending on model flex able base plate, best high back rotation around, toe straps are much better and stay in place better.
Cons, no sole length adjustment, not a fan of the plastic heel loop, no toe or heel ramps to adjust, not as stiff as the Unions base plate wise.
To me either is an awesome choice but when you list it out I think I just talked myself in a set of new Cartels one day. But I do like the union for full on reliability due to simple design also. For an All-Mountain rig sorry to say but Union wins in my mind.