I'd go with the USB device.  SCSI is way overpriced, and hot-swappable SCSI is 
really beyond anybody who's not spending someone else's money.

I don't like parallel devices - Look at Duane's: IDE drives running over a 
SCSI emulation over the parallel port - it's asking for trouble.  Besides 
which, it's difficult to hot-patch them.  Sure, it's linux, you can 
load/unload the drivers and all, but still.

USB, on the otherhand, is meant for hot-plug swapability.  I'd go with that.  
Or firewire - I'm not sure which is more advanced, firewire or USB2.0 for 
Linux.