Is it me or are a number of threads in this forum sounding like Magic The Gathering cards? (yes, I'm a geek, and I know it).
First off, If sorcerors only get the spell, then only sorceror spells would be countered, if we hold to Erevan's idea of only being able to counter spells you know. If it is expanded so that any projectile spell can be blocked, then it has to have a VERY low chance of succeeding... maybe 15-20% at max. Else, it's just too big a buff to sorcerors. Remember, for you MTG fans, you're only allowed four of the 'counterspell' card in a deck, and in tournaments, there's a limit on counterspell-type cards in a deck.
It's an interesting idea, but I think it is just too powerful, and nerfs all casters vs. classes who have this.
So... give it to barbarians.