In my opinion and experiences, most of the apps I have used on both OS versions have minor nuances/differences. Although, on the GS3, I think some apps look better vs my iphone 4 did.
I started with iPhone 3, then 3G, then HTC Incredible running Cupcake (if I remember correctly) then switched back to the iPhone 4, now back to GS3 with ICS. I have to say even after 2 months with my GS3 using Swiftkey 3, what I miss the most is the iOS keyboard. Hands down in my opinion, far superior to ICS, Cupcake and Jellybean 4.2.1 (using this on my nexus 7 now). I really can't narrow it down to one or two differences between the two. Maybe it has just the right size keys with the right spacing that happens to fit my hand size the best. I dunno, but even trying different row, column and key size changes in Swiftkey 3. It just doesn't "feel" the same.