See my discussion of solid state drives with the Mac Pro — the same ideas apply.
The good news is that you can now have dual drives in an iMac: a hard drive and an SSD. But don’t be surprised if you see much better upgrade deals come along by September.
The bad news is that Apple’s offering is for thick wallets: $150 to upgrade to a 2TB drive instead of a 1TB drive (when a fast 2TB drive can be purchased outright for $139!) The price to move to a 256GB SSD (only) is a $600 upcharge, and to move to the SSD plus a 2TB drive is $900.
Add in the Core i7 processor option ($200) and 16GB memory, and check your premises— you can buy a Mac Pro with a 27" display for about the same money and have a better, more flexible system. So unless “cool and hip” is what matters to you, go straight to the Mac Pro.