I had many a droid before I moved to the dark side... and I prefer the 4S over any of them. I loved my droids, and rooting and customizing, but ultimately, the platform isn't as stable. Another user was right, the app stores/market is pretty comparable. Most apps you can find on both. But the execution isn't the same. There are too many software and hardware versions to keep the OS stable on each and every one. They all have their little quirks and bugs that you either live with or upgrade. On my droid, I was constantly looking for the next big/best thing and I found what I was looking for in the iPhone. I haven't looked at another phone or thought of upgrading in months. I did it daily on my Droid(s).
*Sidenote. Siri is great, easy to use, functional for what I need... but SpeaktoIt Assistant on Droid had more features. Just not as easy to access.