I have both the galaxy S2 and the nexus.  I live by the beach.  The S2 screen is slightly brighter and easier to see when on the beach and sun behind you.  You can still see the screen on the nexus.   As experiments I have taken photos with both of same things on beach into the sun and with sun behind and in all tests the Nexus is way ahead of the s2 when sun in front.   Have taken photos around town and home and honestly in everyday photos even though the S2 has 8mp camera and the Nexus a 5mp camera, even magnifying on the computer it is almost impossible to tell the difference between them. 
 With the Nexus I was always accidently calling people when in the phone app and contacts.  I would just touch to scroll and it would ring the contact I had lightly touched.      I have installed go contacts ex and overcome that.
  Call quality I have found about the same on both.
Playing movies and music without earphones the S2 has the louder volume and helpful in noisy places  but I feel the audio on the Nexus is a little clearer.
 The music player on S2 doesnt show the album art where on the Nexus the music player shows the album art
Overall I much prefer the Nexus. It may be slightly larger and really with both together very little difference in width with Nexus longer.  But the Nexus for me is far more comfortable in the hand.
Only unknown is how the Nexus would handle if I accidently drop on concrete which I hope never to find out.
So overall I would go for Nexus.