21.5in vs 27in iMac [late 2012 model]

I've decided to buy an iMac (right before Christmas, I know), and I want some objective opinions from iMac owners about pros vs cons of the 21.5in and 27in new iMac models. Obviously 27in has advantage in size, but 21.5 has cheaper upgrade choices when buying direct from Apple store online. Which one has better/easier upgradability for self servicing? Any overheating or display discoloration issues? For upgrades I'm looking at at least 16gb ram, quad i7 cpu. How is the new 'Fusion Drive"? I know apple just refreshed imac lineup, but if anyone has any experience with the new imacs please chime in. Another question, which external thunderbolt desktop hard drives have you guys used? My budget for hd is ~$600 max, I'm looking for between 2-6 terabytes and with daisy-chaining capabilities. What would you recommend?