The iPhone. Well engineered. Brilliantly designed.
Android. The thicker phone for you. Just another day enjoying the box.
Technically I'm switching from an Android since I've preordered the 5, and am currently using the Galaxy Nexus.
I bought my Nexus earlier this year to get a chance to try out an Android phone, and didn't want to get one that was locked to ATT, or was limited to Gingerbread.
And though I'm switching, I've used an iPhone since the 3G, then the 4. I bought the Nexus to see if I would like Android better. Also so I could give my iPhone 4 to my brother in law who is now driving a truck for a living, and needed a better phone on a prepaid plan that would also allow him to do internet.
My Nexus will be a backup phone for times I feel bored, and useful if either my sister ever needs an emergency phone or I do something not covered by warranty to my iPhone 5.
Going from the Droid X to iPhone 5 (my first iphone). I'm not a 'fanboy' of either OS but personally I'm making the switch for the following reasons, not necessarily in order of importance:
1. More stable and streamlined UI
2. Less buggy apps, hopefully fewer freezing issues
3. Better integration with my MBP using iCloud
4. Light weight and smaller screen. For me, the difference in weight and dimensions trumps any gains in user experience between a 4" and say S3's 4.8" screen
5. I'm not sold on NFC yet
6. Friends & family members who own Apple products with whom I can facetime and imessage for free using unlimited data
7. More robust app store
8. Availability of OS upgrades
9. Better quality construction and hardware
The only thing I will miss is flash, and the only disappointment for me with the iphone 5 announcement is the inability to talk and surf on Verizon. That was almost a deal breaker. Almost.