There are five base configurations, three 21.5-inch options and two 27-inch options. Luckily the differences are pretty straightforward, which should make your choice easier. Let's start with the first two 21.5-inch models.
iMac 21-inch 1.4GHz vs. iMac 21-inch 2.7GHz
1) 1.4GHz Quad-core Intel Core i5 vs. 2.7GHz Quad-core Intel Core i5
2) 500GB hard drive vs. 1TB hard drive
3) Intel HD Graphics 5000 vs. Intel Iris Pro graphics
4) $1299 MSRP vs. $1499 MSRP
For another $200, you receive a significantly faster processor, a hard drive that is doubled in size, and a better graphics card. We recommend the 2.7GHz model in this case if you can afford it.
The other two 21.5-inch models only have a couple differences.
iMac 21-inch 2.7GHz vs. iMac 21-inch 2.9GHz
1) 2.7GHz Quad-core Intel Core i5 vs. 2.9GHz Quad-core Intel Core i5
2) Intel Iris Pro graphics vs. NVIDIA GeForce GT 750M w/1GB
3) $1299 MSRP vs. $1499 MSRP
For an additional $200, you receive an incrementally faster processor and a slight graphics card upgrade.
If you look at the differences between the high-end iMac 21.5-inch and the low-end iMac 27-inch you will notice a $300 price differential for the jump from 21.5-inch to 27-inch. The hard drive is slightly faster in the 27-inch and the graphics card is the same. The rest of the specs are identical.
If you compare the two 27-inch options, again you will see just a few differences.
iMac 27-inch 3.2GHz vs. iMac 27-inch 3.4GHz
1) 3.2GHz Quad-core Intel Core i5 vs. 3.4GHz Quad-core Intel Core i5
2) NVIDIA GeForce GT 750M with 1GB vs. NVIDIA GeForce GTX 775MX with 2GB
3) $1799 MSRP vs. $1999 MSRP
Notice the $200 gap between models for a processor boost and graphics card upgrade. I would probably choose the low-end 21.5-inch or low-end 27-inch because the upgrades to the higher-end models do not seem worth the cost. Instead, you could buy a separate 16GB RAM upgrade for $75-$100 to get a real performance boost that is also cost-effective via Apple Sliced Memory
. Please note that the RAM in the 21.5-inch model is not user-upgradeable.
iMac 27-inch 3.4GHz vs. iMac 27-inch 3.5GHz Retina 5K Display
1) 3.4GHz Quad-core Intel Core i5 vs. 3.5GHz Quad-core Intel Core i5
2) NVIDIA GeForce GTX 775MX with 2GB vs. AMD Radeon R9 M290X with 2GB
3) 1TB hard drive vs. 1TB Fusion Drive
4) 2560 by 1440 resolution vs. 5120 by 2880 resolution
5) $1999 MSRP vs. $2499 MSRP
There are several significant differences between these two models to justify the $500 price increase. First and foremost is the Retina 5K display, which may be worth $500 alone to some users. Throw in the fusion drive upgrade, a better graphics card, and an incremental processor boost and it becomes hard to argue that the iMac 27-inch 3.5GHz Retina 5K Display isn't the better buy between the two models.