From what I have seen (and I'm sure it has been mentioned), games optimized for the One X will not look/perform the same in the Series S. NFS Heat and GRID get a visual downgrade and are downright ugly as of now, and PC3, while running much smoother, shows heavy aliasing and pixelation/blurring in some places. However, I must mention that PC2 looks quite outstanding, and FM6, which has never optimized for the One X, looks very good too. I will test FH4 next to see how it compares with the 1080p/60 fps mode on the One X.
I have not seen screen tearing in the above titles or major drops in frame rate, if any, which is quite welcome, and quick resume is available from an external HD (I thought it was only possible with the internal SSD.)