For a 2017 iMac, USB will be fine.
But... be aware that for Macs that have come AFTER the 2017 -- such as the iMac Pro, 2018 MacBook Pros, and 2018 Mini -- there are reported problems with USB audio due to the t2 chip inside these newer Macs.
But again -- if you're going to be using the 2017 iMac (which DOES NOT HAVE a t2 chip) -- you'll be fine.