I'll tell you what I tell every person who asks this question....
Go iPhone.
But seriously - there are two different philosophies at work here and the real question is which fits your use-case and lifestyle better.
iOS is all about the app experience. The OS and UI are designed to blend into the background - hence why most things are already configured - so the user can experience the apps the App Store has to offer without being bogged down by various settings and performance hiccups. This also cuts down on customization.
Android is all about the OS and UI and how to tweak and make that your own. The apps are really secondary as many things Android does (notification center, widgets) are designed to keep you either out of apps themselves or even off the phone (i.e. quick glance to check a notification light and then switch the screen off). If you are the type of person who spends more time fiddling with settings and performance toggles than actually in the apps, Android is the one for you.
Now, generally speaking you can use both OSes both ways (by jailbreaking an iOS device, or by leaving the settings in Android alone). But clearly each OS has a specific user it caters to.
As far as hardware goes - try to ignore the "well this phone has 13 mp camera and a 2.0ghz quad core". "Specs" really don't do a good job of detailing how effective a smartphone will be for you. Most flagship phones will have processors and enough RAM to do what 99% of the smartphone using public needs/wants to do. That being said, there are some hardware preferences to consider:
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IPS LCD vs. AMOLED: the LCD generally gives a more color accurate picture and offers higher brightness while the AMOLED gives deeper blacks and higher contrast. You should look at phones with both to see which you prefer.
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Build materials: phones like the iPhone and Nexus 4 are made primarily of glass and aluminum while most other Android phones are made of poly-carbonate. All have Gorilla Glass screens, but overall the look and feel of the phone in your hand is what is most important in this area. Again, go to a store and pick the phones up.
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Screen size: Despite what most people will tell you, the iPhone 5 isn't some stretched abnormality....simply a 16:9 ratio (same as the GS3) in a smaller form factor (4"). The Droid Razr M is also the same size. Find out which screen size is preferable for you by holding and using phones with different screen dimensions.
I know this was kind of lengthy, but I hope it helps. Ultimately, like a poster above said, the decision should be yours and based on your own preferences. People who say "that phone sucks" or "this phone is way better" are either comparing an old phone to a new one or are saying so based on THEIR preferences. It can be difficult to wade through the bs....hopefully, whichever you choose, you'll be satisfied. There are so many great offerings out now and on tap for 2013, its a great time to be deciding this!
Happy purchasing!