I don't know if you agreed to the swap or not but I recently upgraded my iPhone 4s to a Samsung Galaxy S III and just returned it today. I had it for a whopping 2 days. It's not that the phone was a bad phone; I was just unfamiliar with the Android Operating System and I find that my iPhone works better for me. This may or may not be the case for you. Before you agree to anything, go into a store and use the display model of the Samsung Galaxy S III and see what you think.
The screen on the Galaxy S III is nice but being a female, it just felt huge in my hands. Again this is my experience with this phone. I like to hold my phone and scroll while reading text with the same hand. I was unable to do that with the Galaxy S III. I had to hold the phone in one hand and scroll with another.
The Samsung Galaxy S III is lightning fast and I never felt a lag in the operating system. On the other hand, I never did with my iPhone either.
Battery life for me in the time that I had it was pretty comparable to my iPhone 4s.
Siri is much better on the iPhone 4s than the Galaxy S III's S-Voice.
They both are really good phones. It just comes down to what you really want and what operating system you prefer. I did ask the sales rep at the store if it was common for folks to want to try a new operating system and they said it was. However, they said that iPhone users have a more difficult time adapting to the Android system than the Android users adapting to the iPhone operating system. Like they said that was in their experience.
Again my recommendation would be to go into a store and try the phone out. See how it fits in your hand and see what you like and dislike about the phone. Good luck.