I've always really liked TSM (now "Spectrum Archive")-- it provides access to such a huge variety of storage devices. I know that the costs of tape haven't made the same meteoric drop as disk, but I'll always recommend keeping it, if only for the reduced electricity costs.
For simplicity, I'd choose the disk storage. It's practically identical to cache, except there's no links to gobble up inodes in your first cache filesystem -- just cheap-and-deep storage, structured similarly to how it's done in cache. It's really the simplest way forward.
And here's another reason to keep it in TSM/SA. This huge cheap-and-deep storage needs to be backed up somewhere. And for a lot of companies that are already using IBM software & hardware, the enterprise backup system of choice for them is TSM/SA.
-JD.