All node sizes offered on AWS and Rackspace come with solid state drives.
SSD is 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 SSD with them soon.
Because SSD isn't available on SoftLayer clusters, the node pricing is lower than the comparable AWS/Rackspace nodes. We also configure the larger SoftLayer node sizes to use larger arrays of disks for more throughput (RAID 0 setup). This mitigates the decrease in disk-access time in nodes that lack SSD.