There are a lot of people who were confused when the Galaxy SII and Nexus were released. People were wondering which one they would get or if there were differences between the two. The Nexus had a bigger screen as compared to the Galaxy SII. For the display, the Galaxy SII is considered to have a better display as compared to the Nexus even if the screen of the Nexus is bigger.
The colors are going to be more vibrant if you would choose the SII. Their chipset is also different. It is expected that since the SII was the flagship phone at that time, it had a higher processor as compared to the Nexus.