Best location (multi-drive system) for VM

Here is my set-up (please do not mistake this information for bragging, it is provided for the purpose of advice):

(i) 17" Unibody MacBook Pro, 2.93, 8GB RAM;

(ii) Boot (internal) drives = 2 x X25-E RAID 0 via AppleRaid;

(iii) external Four Disk Array (2 RAID 0 volumes via AppleRaid) connected via Marvell chipped Esata card.


I'm wondering whether it would be best to place the VM on the External array or on the boot drive. Incredibly enough, the VMs seem to perform better on the external drive (where I also keep the "Users" folders). TIA for any advice.