I'd say go for a Galaxy S2 because I have it and its by far better than the Iphone 4s in my opinion, all the new features in the Iphone 4s have been available for quite a while with the Galaxy S2, its also cheaper. The galaxy also has a larger screen, and is thinner in size!
I'd suggest getting the S2 because the new iphone 5 is supposed to be coming out later this year, so you may regret your purchase of the old Iphone now since a lot of things are supposed to change with the new one!