I'm going to say it right away. Instagram for iOS is better than for Android.
I don't know if it's the 4th Generation iPod Touch's display resolution or that my phone, Nexus One, has limitations but when converting photos on Instagram, the former does a better job than the latter.
Also when I try to convert photos taken by my Nikon D40, they come out better on the iOS verion.
Here are some of my observations:
*Instagram for iOS is on the left and for Android is on the right.
*Photo used was taken on Nikon D40.
Instagram on iOS the whole photo can be scaled to 1:1 ratio, while on Android, you're limited to scale and crop within the photo itself.
The iOS version converts the photo to a resolution of 1231x1231, while on Android, it's 720x720.
Also the photo looks smoother on iOS while it looks jaggy on Android.