There are two basic types of storage devices available on the market today: traditional platter style or hard disk drives and solid-state drives.
For the lowest cost per gigabyte, you can’t go wrong with a hard drive, and they come in truly massive sizes—up to a whopping 8 terabytes. However, they’re slow and their performance is lacking especially with the technological demands we place on computers todays.
For speed and reliability, you want a solid-state drive, also known as an SSD. Because SSDs rely on flash storage, a type of non-volatile memory whose chips retain data without power, they’re lightning fast. But chips are more expensive than hard disk platters and read/write heads, so the $250–$300 that will get you an 8 TB hard drive is enough for only a 1 TB SSD.
So in 2012, Apple came up with an ill begotten compromise: the Fusion Drive. As its name suggests, a Fusion Drive is meant to provide the best of both worlds by providing much of the speed of an SSD along with the capacity of a hard drive. However, in reality it is a Frankenstein monstrosity that should never enter a business office.
While it is true that you might be able to get away with a Fusion Drive in a machine intended for home use – it will never be able to handle a substantial work load.
To make matters worse, many Apple stores' only stock options for the iMac and Mac mini have Fusion Drives. This means that you have to special order a machine to avoid getting stuck with the curse of rainbow wheel which will be your inevitable fate as the Fusion Drive gets stuck switching between the two sides of itself.
One final note. As of this writing, macOS 10.13 High Sierra will not convert a Fusion Drive to Apple’s new APFS file system. We anticipate that will change at some point in the next year, and it is true that APFS might make Fusion Drives a little bit faster though not necessarily anymore reliable.
All that said, if you want the best performance and can afford the cost you should get an SSD. Should you need more space than an SSD can provide, consider using the SSD internally and adding an external hard drive connected via USB 3 or Thunderbolt 3. With today’s technological demands, we cannot in good faith recommend purchasing a computer with a hard disk drive as the primary storage for a Mac unless low cost is absolutely paramount. A hard disk drive's performance just won’t cut it anymore. Which is partly why the Fusion Drive fails to deliver.