I'd guess at the internal RAID 0 being a bit faster. They're slower notebook drives, but they are on SATA2 interfaces. The external RAID uses better drives, but with a 100 MB/s cap imposed by the interface. So the internal drives just need to exceed 50 MB/s each in order to win.
If you want speed, get a fast SSD as an internal boot drive. Use the external drives for mass storage.