A SSD should be the better choice as the faster access and seek times should provide far better performance compared to HDDs in RAID0. A SATAIII connection should allow you to use the full potential of a SSD instead of bottlenecking it to 300MB/s. Another option is to combine SSDs in RAID0 for better sequential speeds but you would see little performance increase in the random speeds.
Consulting with the live HP support may also be a good idea here. 
Captain_WD.