The two are roughly equal in strength and durability, but going by their official stats, Ben has an edge over Piotr in intelligence and fighting ability. I'd concede that Colossus might be a little more agile, but not enough to call it a decisive advantage.
Ben has more tactical experience, and I'd say his will to win is stronger. I give this to the Thing 7/10.
Also, while this is ABC logic, both the Thing and Colossus once faced the Elder known as the Champion in a contest to determine Earth's fate. Neither beat the Champ, but Ben Lasted much longer than Piotr. Though in fairness, Piotr was much younger at the time, both less experienced and weaker.