Depends what u wanna do with the drives.
If u wanna shuttle large files, like video, audio, or fatty Photshop images, the RAID-0 array would work wonderfully .. especially if *reliability* isn't very important. But if u want something to run your OS, apps & swap/page file .. something where a low seek/access time provides max advanatge .. that's where the X15 would shine .. especially if reliability is important .. cuz SCSI drives are more reliable than IDE .. and a single IDE drive is more reliable than a RAID-0 array .. which has no (zero) fault tolerance. One drive dies = you lose everything on the stripe/array.