I have had both.
I had a Samsung Galaxy S Vibrant first, it was a good phone. I loved it, but ended up switching it out to try the iPhone.
I used the iPhone (4S I think) and was a fan of it, it seemed to run smoother than the Galaxy S Vibrant, but little things about the phone began to bug me, I felt that having an iPhone was a bad choice, and regretted getting rid of my Galaxy S.
After smashing my iPhone to pieces, I went back to a basic phone that can only text and call. A few months after I saved up, I went back to the Galaxy S Vibrant, as I got a good deal on it.
I prefer Android, although mine is outdated (Gingerbread), in my opinion it runs better than the iPhone did, and my homescreen is not cluttered with app icons like the iPhone forces, unless you make folders, which is another pain.
The Android is definitely the best in my opinion. Out of all the Androids out there that I've played with ,Samsung seems to have the best ones. My girlfriend has an SIII which I am a bit jealous of. My only complaint is the size of the Note II, my buddy came to town this weekend and has one, it felt so uncomfortable playing around on it.
Verdict is, my opinion = Samsung.