We have EMC DMX-3, CX4-480, CX3-40 & Centera arrays plus a few racks of IBM DS4800s. All of our storage is allocated as part of a tiering structure. The DMX-3 hosts our Tier1-4 storage, the Clariions host Tiers 5 & 6 & the Centera is our archive tier. The DS4800s are used by our developers for dev, test & QAT.
We grew from 2 Dell CX-500 arrays (our first SAN), adding in the CX3-40 the next year and then alll of the other EMC gear in just a matter of a eighteen months after that.
I hope that is some help for you.
In our case, we tend to buy arrays that would fulfil the needs of a project over its life. So we might buy a 120TB capacity array, with 40TB loaded, but then buy another 200TB capacity array which also might be 40 TB loaded.
But we do try to stay with one vendor, mostly to preserve the investment in training.