The basic difference here is that Jonas's first experience with snow is pleasant while his second experience is not.
In his first experience with snow, in Chapter 11, Jonas enjoys himself. He is sledding and it is exhilarating and fun. But then, in Chapter 14, he is given another memory that involves snow. This time, he is sledding but he loses control and hurts himself quite badly. This is a big difference. The second time, the snow is used to teach Jonas not about fun and pleasure, but about the sort of pain that is rarely experienced in their community.
The Giver tried to explain to Jonas the experience of sledding downhill. Jonas, however, was puzzled. He had no idea what The Giver was talking about. There was no snow in his community. There were no hills there, either.
In order for Jonas to understand, The Giver gave him the memory of snow. This memory was a positive one as "tiny, cold, featherlike feelings peppered his body and face." He sled down the snowy hill and a "breathless glee... overwhelmed him." The snow was powdery and ideal for sledding.
With hesitation, The Giver decided that Jonas needed to experience memories of pain. He made the decision to "start with something familiar." He chose another memory with snow. This time, the snow was not fluffy and powdery. It was "hard, and coated with bluish ice." Something felt different to Jonas as he sled down the icy hill. Jonas lost control of the sled and it "hit a bump in the hill." He fell hard and "could hear the crack of bone." He was filled with pain after he landed on the hard ice. The pain felt like fire to him. He was bleeding and he threw up.
The first experience was one of joy and excitement. The second experience was one of pain and fear.
along with what pohn said, he was also more focused on the color of the sled the 2nd time since the Giver told him to think about the color since he was begining to see in color