Either one will work fine. They are both excellent LED fixtures. The differences come down to the features that each one has, and how the mounting your going to use will look over your tank.
Generally, I'd say the Kessil is a little less expensive, and the Radion can do a few more tricks, but these are really not major considerations.
Lastly, both of these fixtures are pendant type fixtures and tend to direct light straight down. Normally,this often means to get the tank looking good, with out any dark ends or corners, you usually need more than one fixture. However, you should be able to get away with one because a Red Sea Reefer Nano is only 18 inches long.