I don't think counterspell should be a spell at all. It should just be a type of action that any spellcaster can take. For example:

Counterspell: As a reaction, you can attempt to counterspell a spell being cast by a creature within 50 feet of you. You engage in a contest with the caster, opposing your magic ability check against the caster's. If you win, the spell has no effect. If you beat the caster's check result by 10 or more, you can reflect the spell back upon him, as if you had cast the spell and the original caster was the primary target. On your next turn, you cannot take an action.

To take this action, you must expend a prepared spell or unused spell slot of equal or higher level of the spell being countered or an equal number of Willpower points (you know how powerful the spell is once you choose to take this action). If you are a warlock, you can expend an unused favor instead, but only if your warlock level is at least twice the level of the spell being countered. If you don't have a powerful enough spell or enough Willpower points, you can't attempt to counter the spell, but you don't lose your next action.