I sure hope Brassard is the better player. I hope he's the significantly better player, because if he isn't, then the Sens made a terrible terrible trade.
No GM in their right mind would trade a 23 year old center for a 29 year old center if the 23 year old was as good or better than the 29 year old. From my viewings of both players, Brassard looks to be the better player of the two. However, it doesn't look to be by a big enough margin to justify giving up 6 years in age, Zibanejad's RFA status, and the potential that Zibanejad possesses.
In 3 years Brassard will be a 32 year old UFA, while Zibanejad will be a 26 year old kid just entering his prime, likely already having been signed to a long-term contract. Given the Sens disinclination to spending money, it's a good bet that they let Brassard walk in 3 years, given that he'll be 32 and probably looking for a contract that will carry him to retirement. In 3 years the Sens could very well have nothing to show for this deal, while the Rangers will have a 6'2, 220lb top 2 centerman just entering his prime at age 26. That's not a deal a team should make just because of handedness and chemistry.
And if the deal weren't bad enough already, Dorion made sure to throw a 2nd round pick in the deal, just cause.