SSD drives are the fastest storage media at this time but if you put the disk behind a slow interface then the performance will suffer. What I/O ports does your system have? E-SATA is optimal but Thunderbolt and USB3 are not bad, FireWire 800 is OK, but USB2 and FireWire 400 are getting pokey.
Striped RAID arrays via E-SATA are good and give a lot of storage for the price.
It really depends on your I/O ports and your capacity needs are.
HTH,
-Kirk