ssd definately, but i would stay away from two ssd's in raid at the mo.
problem is, ssd's tend to degrade in performance over time (once the flash cells are written over the first time after fresh or something...). recently, TRIM has been introduced into the firmware of SSDs (dunno how it works, so i wont go into details) to counter this, so you shouldnt notice the performance degration any more. only problem is, you cant use TRIM in raid ... yet. you set up SSDs in raid, they'll be blazing fast, but then they'll get worse over time.