I am currently using an Ativ S and I recently bought my son a S3 so I have a pretty good idea how the panels compare.
As already stated they are either almost identical or are identical. The main difference is how WP and Android handle screen brightness. If you put them both in auto mode then they behave the same. The big advantage Android (i.e. the S3) has is that you have a slider to adjust the brightness manually whereas WP (i.e. the Ativ S) has three settings in manual low, medium and high.
The main thing to note is that the Ativ S panel is good and vibrant and plenty bright enough – especially of you set it to high. The one disadvantage it has is the typical poor amoled performance in strong direct sunlight.