You said the magician has "Cancel Channeling sphere". I assume you meant "Dispel Channeling Sphere" because "Cancel Channeling" isn't a sphere and doesn't have a radius. Also, "Cancel Channeling" only blocks spells that target the caster, in my opinion that wouldn't trigger here at all.
"Dispel Channeling Sphere" comes in versions I, V, and X with varying radius (10, 50, 100).
Spell Shield II.... hmm, that has an area of effect of "caster" and doesn't dispel other spells, it only subtracts from spell attack rolls and allows parrying of one spell attack.
So... Cancel Channeling vs Spell Shield II: there's no effect. Neither spell effects the other. Both remains in effect if the casters target each other with attack spells.
Or... Dispel Channeling Sphere vs Spell Shield II: Spell Shield II may be dispelled. It would be reasonable to subtract the 10 from the BAR roll for Dispel Channeling Sphere, but Spell Shield II won't under any circumstances get rid of the Dispel Channeling Sphere.
Or... Dispel Channeling Sphere I vs Dispel Essence I (picking a likely spell that dispels other spells in a radius), both have a 10' radius, I would give them simultaneous chances to dispel each other.