Card reader question
The next time you have a card with a lot of images on it, why don't you try importing the images twice (once using each reader), and timing it both ways?
That will certainly be part of my learning curve for the new camera / cards. I am still curious if the group has a preference for one reader over the other (outside of the desktop clutter created by the USB card reader).
Outside of the "desktop clutter" all that's left is what Tom said, which one will perform better.
Your 2010 iMac supports the highest current SD spec, which is the SDXC type of SD cards. I would ditch the "desktop clutter" and enjoy the convenience of your builtin SDXC card reader. One of the biggest reasons for owning an iMac is to get rid if "desktop clutter."