Ive used my mates SII, its a great phone to be sure, but honestly in everyday usage terms theres almost nothing between them. Theres some factors i prefer about the SII, but none of them are really enough to make me wish i had one over my NS.
That may change with the arrival of the Nexus prime. However, ultimately everything becomes obsolete, thats why i purchased the NS, hopefully to avoid some of that. However, i do think that Google really should have made the NS a dual core device, even if it delayed it slightly.