Got an Nexus 4, had an S2.
The nexus 4 is a great phone, does everything very well and the stock android user interface won me over. One of my pet hates with the samsung was the wrapper they use for their phones, a lot of software I didn't use and was very gimmicky feeling. Admittedly I do use a lot of Google apps, so that swayed my experience.
Depends what you need from a phone, and whether you are setup to move away from iOS easily, can be a pain initially transferring musuc etc etc