All node sizes offered on AWS and Rackspace come with solid state drives.
SSDs are currently unavailable on any of our SoftLayer node sizes, due to a limitation on hourly virtual instances with that provider. We are communicating with SoftLayer about this limitation, and hope to offer SSDs with them soon.
Because SSDs aren’t available on SoftLayer clusters, node pricing for SoftLayer clusters is lower than comparable AWS/Rackspace nodes. We also configure larger SoftLayer node sizes to use larger arrays of disks for more throughput (utilizing a RAID 0 setup). This helps mitigate the increase in disk-access time.