[secorp]

We run a small specialized storage company and the things that seem to matter most are: storage capacity, availability, reliability, transfer rates for both current data usage and new data addition.

40Tb can be handled pretty well by S3 and other storage services and they have pretty good pricing information to model your costs. Note that they don't (yet) provide very specific SLA's for data availability, so keep that in mind when designing your system.

Maintaining your own drives with some sort of redundancy (RAID, automatic copies, etc.) or using something like (bias alert) our open-source project http://allmydata.org which is effectively a software RAID layer both require some IT and systems energy, so this has to be bundled into your operational costs if you choose that route.

Just to emphasize what others have mentioned, it is important to incorporate the new data influx rate into your model. If you are successful, 40Tb this year might turn to 120Tb next year, so make sure that your cashflow model can support the underlying cost of whatever system you choose.