As someone who switches back and forth at times between a 521 and a GS3...here's my take on it. As with all this, its just an opinion, and it is in no definitive way right or wrong.
The things that stand out about Android to me are:
More customization options (Different apps for just about every purpose, SMS, theming status bar, roms, etc)
All apps more or less head there or iOS before anywhere else.
Depending on the device, you could get great support for updates. Or none at all. Device and carrier specific. The GS3 is a perfect example of great updates. Tons of updates have happened for this phone.
Now we move on to WP8/WP8.1:
Things I like....
Camera. I don't know about other phones, but my 521 has an awesome camera. It blows away the GS3's in my mind.
Less lag. Yes, there is an occasional stutter, but on my GS3 it was always lag lag lag. Even unlocking the phone.
Simplicity. Everything I want to do just works. Web browser? No problem. Amazing camera? Sure thing. Smooth transitions? 99% of the time.
So really, while android might have the upper hand in some areas, my 521 has it bested in others. Ones that are more important, to me.
Sure, I can't play candy crush (Is that what the popular game is these days?) but that doesn't matter to me.