GC-68. The SK-6 *usually* doesnt do all that well with an 80mm fan for several reasons. One, it needs a lot of airflow, and that airflow is better when its concentrated. So a 30cfm 60mm will usually perform better than a 30cfm 80mm fan. Also, with an 80mm fan, at least 25% of the air being flowed isnt even between the fins. Every so often there will be fluke and someone will see better resuts with a 80mm fan, but chances are its a much higher cfm fan than the 60mm they were using. If not, well then, they are lucky.
Since the GC-68 was designed with a 80mm fan in mind, you dont have to worry about lost airflow or anything. Its a lesser peforming heatsink than the SK-6, but when you cripple a SK-6 with an 80mm fan, they're fairly close. With that in mind I would purchase the GC-68.