SSD Cable or Enclosure?

I have an iMac 21.5in (Late 2015) that is slow. I’ve been using it more than often because I am working online now (cause’ of the COVID-19 thing). I recently got a Samsung 860 EVO I’ll be using as an external boot drive because the internal HDD is now running a little slow (write speeds of under 20mb/s and read is a like under 25mb/s) and I can’t have more than one window open or it gets slow. To connect the SSD should I use a SATA to USB 3 cable or a USB 3 enclosure? Thanks