There should be something lase between Nexus & Alfine. I've tested this weekend the AC01 and AC02. The price difference here in the Netherlands is 1000 euro fix between the 2. I know there is the frame/fork and belt which are different. There might be something on the transmission too.
Both ride smooth. However, you feel the difference in the AC01. It's lighter and turns a lot easy. I didn't actually felt a big difference in the gear change, however, I've just tested them for 10 min each