I've used Rackspace cloud both when it used to be called Slicehost and after the full transition to the Rackspace brand. In my experience past and present, you get a better deal dollar for dollar with Linode (better disk, network, and cpu performance). Personally, the only reason I'd use Rackspace cloud would be as a part of their hybrid offering.
Good call - lease a bigger 16-core 32GB box such as your database server on their managed hosting platform (not cheap), and then just spin up cloud instances connected to that server using RackConnect which seems to be some fancy, cumbersome routing/vlan monstrosity.
