Seems to me that there are 2 big benefits over sizeup (though i'm basing this entirely on their website, i've not used it)
1 - With sizeup you have to manually move your apps around. With this, new windows are automatically tiled (and you can then move things around afterwards with the keyboard shortcuts)
2 - This seems like it supports more complex layouts (with various layouts built in)
That is generally correct. I wrote a longer comment about this but SizeUp is about manually managing windows and Amethyst is about letting software manage windows for you. It's generally a matter of personal preference which you prefer.
1 - With sizeup you have to manually move your apps around. With this, new windows are automatically tiled (and you can then move things around afterwards with the keyboard shortcuts)
2 - This seems like it supports more complex layouts (with various layouts built in)