Past Pre 2 user here. I have been with webOS since 2009 until now. However, during the webOS life that I had, I also owned an iPhone 4 and a Samsung Epic 4G (I used the Pre 2 for work purposes).
While I thought webOS was the best operating system, I will say that Android is BY FAR the most powerful. Things that you have to root webOS to do you can do on stock Android. In fact, the only reason you should really root an Android device is to do things like get a free mobile hotspot or overclock the processor. Every other customization that I have wanted to do on Android is possible just from it being stock.
iOS 5 have caught up to Android in many ways, especially with its amazing voice command structure. However, from the Videos I have seen on Ice Cream Sandwhich, Android is looking to be the best overall operating system. If the video of ICS that is floating around the web is real, I think I have my dream phone.
I purchased the Motorola Droid Bionic a few weeks back. I returned it because of this Nexus Prime device. I am really looking forward to it. I think you will be better off in the Android world. It just really makes you feel like your phone is actually YOURS. iOS looks the same on every iPhone you will ever see. But on Android, I can literally change the way my entire device works within seconds. It amazes me.