Think I would go frost to.
But if you wanna stick with your Bryntroll, you should stay unholy. and then you should use a frost sub-specc, down to Icy Talons in the frost tree.
With BiS Unholy>Frost but if you are in a more casual guild and unlikely to get BiS items like Shadowmorune then go Frost. Frost brings the nice melee haste buff if you do not have an Enchance Shamman but Unholy will boost the casters DPS. Either way both Frost and Unholy will benefit people in your raid as you more then likely have a Marks Hunter to provide the AP that Blood would bring.