I have both on our systems,... 2 500 Gb HDs as RAID 0, where leaves all the Cat original content and ArtBetas, VideoTraxx etc.... and a SSD drive for Custom content, or playing back more layers.
With the SSD, you can get easily 10 layers (sometimes more) of SD content and a couple layers of HD content.
On the 500Gb RAID, you can get tops 4 layers of SD content and 1 layer of HD content.
Of couse this is generalizing the system, but is just to give you an idea of the difference. Also those numbers are not real for evrething, and depend on the bitrate of the movie, codecs, alll that things we leave... That is just to give you and idea of the difference.