Apple recently changed their iMac lineup, adding a new 21.5″ Retina 4K iMac (Late 2015), plus a refreshed 27″ 5K Retina iMac (Late 2015) and non-Retina 21.5″ iMac (Late 2015).
All of these Late 2015 iMac models offer a 1TB Fusion Drive as a storage option, however Apple has significantly reduced the amount of flash storage included with this option from 128GB to 24GB. The 2TB and 3TB Fusion Drive storage options still come with 128GB of flash storage.
Because of this reduction, I’m recommending avoiding the 1TB Fusion Drive altogether, and instead going with a larger Fusion Drive or SSD storage option, especially on iMacs upgraded to 32GB memory.
Also of note, the refreshed non-Retina 21.5″ model now has its memory soldered to the motherboard, preventing after market RAM upgrades. The Retina models still have two memory slots that can be upgraded by the user.