The primary consideration is what you need the extra space for. If your current internal drive/SSD is almost full, what types of files are making it full? Myself, I do not have many photos on either of my Macs (a late 2012 Mac Mini with a 256 gig SSD, and a mid 2913 13" MacBook Air with a 252 gig SSD). I also don't have a "ton" of music on either one, and also not many "data" files (Word documents, Excel spreadsheets, etc.). I do have quite a few movies and TV series, but I have them on an external SSD, and that suits my purposes fine. I also have quite a lot of other "stuff" on my external SSD, but accessing such information from my machine works well.
So, if the need for the extra space is "only" for photos/music/"documents", etc., an external HDD could be the way to go. You would then need to decide on 1)the capacity of the drive, 2) its rotational speed, and 3) the connecting interface. Any/all of those will influence the speed at which you are accessing the information and writing to that drive.