What are the deals you are looking at? I would think you could get a Galaxy S3 for similar money to the 4S...
Otherwise its a tough choice for me.... I think I would get the 4S even though I think the Galaxy is the better phone (because of Android) purely because the S2 has been superseded now and the 4S will be worth more to sell on later.
Depends on your use and how techy you are though.
Also in my experience the 4S is a lot more expensive than the S2 now, so if you have found them for the same price, you've either found a very good deal on the iPhone or a poor deal on the S2.