Be sure to check prices. I was buying enclosures and ssd/hdd separately, but recently noticed that it's actually cheaper to just buy an external ssd/hdd drive, even from the same OEM (f.ex. Samsung). By the time you buy the enclosure, the drive and the cable, you might be paying more than just getting an external drive with everything included. Cost - for larger capacity drives, you'll pay more for SSD than an HDD; however, watch out for "cheap" drives on sale from unknown brands and sellers, there is a lot of fraud out there where the advertised capacity is fake, especially on Amazon - stick to known brands and sellers. Speed - with modern SSD/HDD, the speed limitation is going to be the ports on your iMac, the drive enclosure and cable. Reliability - it seems that SSD are pretty reliable (again, from known OEMs such as Samsung, Crucial etc. and known sellers) although it is said that when they fail, they tend to give little warning. Modern HDDs are pretty reliable (again, from known OEMs) and cheaper at larger capacities. But there are regularly mini-scandals regarding some really bad batch of drives from Seagate or WD or whatnot. Also, be aware that many known "brands" simply put their own enclosure and badge and the drives inside are from the same handful of OEMs. Always google for the latest reliability alerts, like the recent disaster from Sandisk's external SSD drives.