This is more a question for Android users really, especially Samsung Galaxy S3 owners who have had an iPhone. Please let's not go down the 'I have this one' so clearly it's better route. Although always happy to hear of happy purchases
I'm very familier with Apple and the iPhone. I have an iMac, iPod Touch, iPad1 and iPad2 and my laptop is a Macbook Pro. My wife has an iPhone 4s. As such it's pretty clear I like Apple products but most importantly I have a pretty big collection of iOS Apps that would immediately be available to an iPhone. THIS is the bit that has me leaning to the iPhone - the Apps.
Not wanting to be a slave to Apple what would be the downsides to buying into Android? I've had a brief play with an S3 and it's very nice, I'm keen. My work environment is Microsoft so I don't use Mail, Contacts or iCal to manage my organiser. All I need a phone to do is Sync with Outlook, run all the Apps I use most often (most are cross platform and cheap so should be fine), be fun to use and a good business tool.
Basically, will I regret not having bought an iPhone due to some reason I can't think of.