Compare The 21.5-Inch & 27-Inch iMac

The Apple iMac comes in two different models, the 21.5-inch and the 27-inch, both of them are similarly equipped but still have many differences between them.
The base model of a 21.5-inch iMac starts at $1,099 with a 1920 x 1080 pixel display, 1.4 GHz dual-core i5 processor, 8 GB RAM, 500 GB hard drive, and Intel HD 5000 integrated graphics. The upgraded model that costs $1,299 has a 2.7 GHz quad-core i5 processor, 8 GB RAM, 1 TB hard disk drive, and Intel Iris Pro graphics. Then the iMac that is top of the line 21.5-inch iMac starts at $1,499 with a 2.9 GHz processor and faster Nvidia GeForce GT 750M graphics, with 1 GB of dedicated video memory.
The 27-inch iMac starts at $1,799, it has 2560 x 1440 display – the same size and resolution as Apple’s Thunderbolt Display. A 3.2 GHz quad-core i5 processor, 8 GB RAM, and a 1 terabyte (TB) 7200 RPM hard disk. The $1,999 model has a 3.4 GHz processor and faster Nvidia GeForce GTX 775M graphics. More detailed specs of both the 21.5-inch and 27-inch can be found at the iMac specs on the Apple website.
 
Also read the Mac Buyers Guide for buying an iMac, here:
iMac Buying Guide
 
Upgradable versus non-upgradeable RAM options
8 GB comes standard on all Mac models. The 21.5-inch iMacs can be upgraded by Apple up to 16 GB of RAM while the 27-inch models can be upgraded up to 32 GB of RAM. The reason for the big difference is because the 21.5-inch iMac has two RAM slots, while the 27-inch iMac has 4 RAM slots.
The main difference between the 21.5-inch and 27-inch iMac is the ability to add RAM to the computer after purchase. Only the 27-inch model has easily user-accessible RAM that you can add more RAM to yourself. While on the 21.5-inch iMac, the amount of RAM you get when you first by the Apple computer is the amount you’ll have for the life of the iMac.
The more RAM a computer has allows for it to run more apps and software together without reducing the performance. Those you plan to use Photoshop, music editing, movie editing or any demanding software it’s recommend to upgrade your RAM size.
 
To see which Mac upgrades you should get for your computer, read: Mac guide for CPU vs RAM vs SSD upgrades
 
Who should buy the 21.5-inch iMac?
The 21.5-inch iMac is the lowest-priced iMac available, and is a great computer that has a lot of power. It allows you to run many different apps and software at the same time. With just a few upgrades, you can have a high powering desktop computer with the 21.5-iMac. The smaller size of the 21.5-inch iMac makes it better suited to environments where space might be restricted – smaller desks, counter space and small apartments. The lower cost comes with a few shortcomings – a slower hard disk, fewer customization options – but don’t let a limited budget or concerns over space dissuade you from the iMac.
21.5-inch iMac:

 
 
Who should buy the 27-inch iMac?
After the Mac Pro, the 27-inch iMac is the fastest Mac has to offer. With a fast processor, large storage and lots of desktop space make the iMac the most versatile and powerful Mac you can buy at a price below $3,000. The power and screen size is not something everyone needs. But for those professionals or regular users who want a premium top of the line computer from Apple, the 27-inch iMac is a great machine to have for maximum performance.
27-inch iMac: