I have both an iPhone 4S & a Samsung S II, each on their own line.
I've owned every model of iPhone and several top of the line Androids since the day Android was released via T-Mobile.
They are both excellent in their own ways. I'm currently using my SGS II as my primary smartphone for several reasons.
First is it's huge gorgeous display. Traveling for work & pleasure I end up doing a lot of my web work via the phone. The larger display is so much nicer, makes typing easier, and less scrolling makes a big difference.
I use Gmail, Google Docs, Maps, Tasks, and for that reason alone an Android phone is terrific. It's a real time saver. All the Apps I use are available for both platforms. In fact now that Android is so popular they have some great apps that still aren't available for iPhone.
It's all a matter of personal preference, for me I'd really like to see Apple catch up since I have several Macs as my platform of choice.