If you mean which reader to buy to use with the copilot, then, as I don't use a copilot, I'd guess the USB 3 version....
As to buying for an iMac. The USB 3 version will work on any iMac, but will be slower (and cheaper!) than a TB3 version. If your iMac has a tb2 port and you buy the TB3 version then you'd also have to factor in the cost of a TB3 to TB2 connector cable, and you'd only get TB2 speeds. IIRC, TB3 ports will only be on iMacs from the latest refresh (2018).